Closes gh-8824

Closes gh-9040

Closes gh-9039

Closes gh-9038

Closes gh-9037

Closes gh-9036

Closes gh-9035

Closes gh-9034

Closes gh-9033

Until we can use the Netty BOM (see gh-9021), this change adds all
Netty modules as managed dependencies so as to avoid version
clashes between managed and unmanaged Netty dependencies.

Fixes gh-8553

This commit changes the default file extension for Mustache templates,
from `.html` to `.mustache`, which is the file extension used in the
official reference documentation and by most IDE plugins.

Fixes gh-8997

Thymeleaf 3.0 implements the Spring 5.0 view infrastructure for WebMVC
and the new WebFlux framework. This commit adds auto-configuration for
the WebFlux support.

In that process, the configuration property for `spring.thymeleaf` has
been changed to add `spring.thymeleaf.servlet` and
`spring.thymeleaf.reactive` for MVC/WebFlux specific properties.

Now that the `spring-boot-starter-thymeleaf` does not only support
Spring MVC, the transitive dependency on `spring-boot-starter-web` is
removed from it.

Fixes gh-8124

This commit ensures that a primary JSR 303 and Spring Validator will be
exposed if the auto-configuration kicks in. As `LocalValidatorFactoryBean`
exposes 3 contracts (JSR-303 `Validator` and `ValidatorFactory` as well as
the `Spring` validator one), this makes sure that those types can be
injected by type.

`LocalValidatorFactoryBean` exposes 3 contracts and we're only checking
for the absence of a `javax.validation.Validator` to auto-configure a
`LocalValidatorFactoryBean`. If no standard JSR validator exists but a
Spring's `Validator` exists and is primary, we shouldn't flag the
auto-configured one as `@Primary`. Previous iterations on this feature
have made sure that we'll auto-configure at most one
`javax.validation.Validator` so not flagging it `@Primary` is no problem.

This commit also restores and adds tests that validates
`ValidationAutoConfiguration` will configure a JSR validator even if a
Spring Validator is present.

This effectively fixes gh-8495 in a different way.

Closes gh-8979
Closes gh-8976

This commit reverts c9561f03 and 69a8c0d8 and effectivly fixes gh-8979 but
reintroduces the issue reported in gh-8495.

* pr/9020:
  Upgrade to Spring Kafka 2.0.0.M1

Closes gh-9020

See gh-9000

Add a test to ensure that the new binder can bind correctly to a
Set.

Closes gh-1415

Closes gh-9000

Remove the deprecated relaxed binder code that has now been replaced
with the configuration properties binder.

Closes gh-9000

Add a test to ensure that the new binder can bind correctly to a
Map<String,String[]>.

Closes gh-3789

Add a test to ensure that the updated configuration properties work
correctly binds environment sources in the `/info` endpoint.

Closes gh-7388

Add a test case to show that the updated configuration properties binder
correctly binds environment variables to complex maps.

Closes gh-8902

A test to ensure that the updated configuration properties support
correctly expands placeholders using mapped properties.

Prior to the updated support an `application.properties` file of the
form:

	my.prefix.some-url=http://github.com
	my.prefix.login-url=${my.prefix.some-url}/login

Would not correctly expand the placeholder against the environment
variable `MY_PREFIX_SOME_URL`

Closes gh-2496

Remove the `relaxed` attribute from `@ConditionalOnProperty` and instead
rely on the direct configuration property source relaxed name support.

Closes gh-9003

Extend `BindFailureAnalyzer` to support the exceptions thrown by the new
configuration properties `Binder`. Origin information is now also
reported when available.

Closes gh-8934

Update `spring-boot-samples` to use the new configuration properties
support.

See gh-9000

Update `spring-boot-devtools` to use the new configuration properties
support.

See gh-9000

Update `spring-boot-actuator` to use the new configuration properties
support.

See gh-9000

Update `spring-boot-autoconfigure` to use the new configuration
properties support.

See gh-9000

Update `spring-boot-test` to use the new configuration properties
support.

See gh-9000

Update `spring-boot` to use the new configuration properties support.

See gh-9000

Remove `ConfigurationProperySource` from `PropertySourcesPropertyValues`
so that the deprecated code to continues to work.

See gh-4910

Update `SpringApplication` to automatically attached the
`ConfigurationPropertiesSource` when a `SpringApplication` runs.

See gh-4910

Create a new `Binder` class specifically designed to bind properties
from one or more `ConfigurationPropertySources` to an object.

The binder provides a replacement for `RelaxedBinder` and attempts to
fix the limitations of the previous solution.

Closes gh-8868

Add a new abstraction that represents a source for configuration
properties. The new source is similar to the `Environment` abstraction
provided by Spring Framework but follows a stricter set of rules.

The `ConfigurationPropertySource` provides a uniform view onto a source
and will help to move responsibility for accessing properties in a
"relaxed" way away from the caller.

The `ConfigurationPropertyName` class enforces strict naming rules
that callers must follow when accessing properties. Configuration
names are lowercase, dot separated and may contain dashes. In
addition "indexed" name elements may be defined by using square
brackets.

Mapping is provided to existing Spring PropertySources implemented with
the relaxed rules that users are used to. For example the configuration
property `server.local-host` can still be written in a property files
as `server.localHost` or in an environment variable as SERVER_LOCALHOST.

Closes gh-4910