Update `PropertiesLauncher` so that classes can be loaded outside of
`BOOT-INF/classes`. You can use a subdirectory, or the root directory
of an external jar (but not the parent archive to avoid issues
with agents and awkward delegation models).

Fixes gh-8480
Closes gh-8486

Relax `SpringBootTestContextBootstrapper` rules so that a test can
specify an `ApplicationContextInitializer` and still have
`@SpringBootConfiguration` detected.

Prior to this commit detection would not occur because it's possible
that an initializer _could_ register configuration. This scenario is
actually quite unlikely to occur, certainly less likely than wanting to
use an initializer in combination with auto-detection.

Fixes gh-8483

Update `ValidationAutoConfiguration` and `WebMvcAutoConfiguration` to
ensure as much as possible that only a single Validator bean of each
type is registered.

Validation auto-configuration now does the following:
- If no validator is found: Registers a `LocalValidatorFactoryBean`
  (providing both Spring and JSR validation)
- If the user defines a Spring & JSR validator: Backs off
- If the user defines only a JSR validator: Adapts it to a Spring
  validator (without exposing another JSR implementation)

WebMvcAutoConfiguration auto-configuration has been updated to make
MVC validation follow common Spring Boot patterns:
- If not validator beans are found (due to the user excluding
  ValidationAutoConfiguration) a new `mvcValidator` bean will be
  registered.
- If a single validator bean is found it will be used for MVC
  validation.
- If multiple validator beans are defined it will either use the one
  named `mvcValidator` or it will register a new `mvcValidator` bean

Any automatically registered `mvcValidator` bean will not implement
the JSR validator interface.

Finally, it is no longer possible to provide an MVC validator via a
`WebMvcConfigurer`.

Fixes gh-8495

Closes gh-8646

See gh-8939

See gh-8938

This commit reverts 65a82576 as the Couchbase driver must be upgraded from
2.3.x to 2.4.x

See gh-8939

This commit reverts 2deab52a as the Couchbase driver must be upgraded from
2.2.x to 2.4.x

See gh-8938

Closes gh-8939

Closes gh-8938

* pr/8935:
  Upgrade to Spring Integration 4.3.9.RELEASE

Closes gh-8935

Closes gh-8712

* pr/8930:
  Upgrade to Spring AMQP 1.7.2.RELEASE

Closes gh-8930

* pr/8929:
  Upgrade to Spring AMQP 1.6.9.RELEASE

Closes gh-8929

Update SpringBootTestContextBootstrapper to detect `@ContextHierarchy`
annotations so that only the last child creates a
`WebApplicationContext`.

Prior to this commit a context hierarchy would start two embedded web
servers which is inconsistent with the `SpringApplicationBuilder`
behavior.

Fixes gh-8311

Allow `PathExtensionContentNegotiationStrategy` to be bypassed by
actuator endpoints. Prior to this commit calling `/loggers/com.aaa.cab`
would return a HTTP 406 response due to `.cab` being a known extension.

Fixes gh-8765

* pull8924:
  Fail fast on getKeyFromServer() failure

Update `ResourceServerTokenServicesConfiguration` to fail fast if the
`getKeyFromServer()` call fails. Since the key is part of the singleton
`JwtAccessTokenConverter` bean there is not real way to refresh without
restarting the application.

A hard failure seems preferable to an inconsistent state.

Closes gh-8924

Closes gh-8497

* pr/8906:
  Polish "Fix statsd metrics collection for names with ":""
  Fix statsd metrics collection for names with ":"

Closes gh-8906

Statsd server is ignoring malformed metrics. This change introduces
a basic sanitizing to metric names for avoid losing those metrics.

See gh-8906

Rename `AbstractTemplateAvailabilityProvider` to
`PathBasedTemplateAvailabilityProvider`.

Closes gh-8913

Closes gh-8672

Closes gh-8903

Closes gh-8592