Closes gh-21085

Closes gh-21087

Closes gh-21088

Closes gh-21084

Closes gh-21092

Closes gh-21091

Add how-to documentation describing how health information can be
exported to a Micrometer Gauge.

Closes gh-18329

Closes gh-21090

Fixes gh-20665

Closes gh-21083

Closes gh-21082

Closes gh-19782

* gh-21069:
  Polish "Improve property name in thrown Micrometer ValidationException"
  Improve property name in thrown Micrometer ValidationException

Closes gh-21069

See gh-21069

See gh-21069

Update `JarFile` to track when the instance has been closed and throw
an exception in the same way that `ZipFile` does.

Closes gh-21072

Closes gh-21074

Fixes gh-21073

Closes gh-21075

Closes gh-20895

Prior to this commit, when the Maven plugin copied
spring-boot-loader.jar to a repackaged archive the timestamps of class
files were preserved but the timestamps of directories were not
preserved. This resulted in the directories having a current timestamp.

This commit copies the directory timestamps from spring-boot-loader.jar
to the repackaged archive and adds tests to verify the proper
behavior.

See gh-20927

Update `JarURLConnection` to ensure that when connections are opened
a new copy of the JarFile is provided.

Prior to this commit, a single `JarFile` instance was shared which meant
that it could be accidental closed if accessed via
`JarURLConnection.getJarFile()`. If the underlying jar file is closed
then it's possible for a `NoClassDefFoundError` to be thrown if running
on JDK 11 with an active `SecurityManager`.

Closes gh-17796

Closes gh-21071

Closes gh-21070

Update `WebMvcAutoConfiguration` to automatically apply cors
configuration to the `WelcomePageHandlerMapping`.

Fixes gh-21048

* gh-21067:
  Polish "Update validation of Micrometer configuration"
  Update validation of Micrometer configuration

Closes gh-21067

See gh-21067

See gh-21067

This commit reviews the current timeout options. It creates a connection
and request sub-namespace to separate concerns a bit more.

Closes gh-19673

Closes gh-21065

Closes gh-21049

* improve-es-config:
  Polish
  Improve Elasticsearch RestClient customization capabilities

Closes gh-20994

This commit harmonizes the change made to @DefaultValue to the
annotation processor. If such annotation is added to a scalar value with
no value at all, no default value is produced.

Closes gh-18917

See gh-20994

At present, RestClientBuilderCustomizer allows general customization of RestClientBuilder.
This is troublesome for users that want to customize `HttpAsyncClientBuilder` and
`RequestConfig.Builder` since those are set on the `RestClientBuilder`. By customizing
those two builders user lose out on Spring Boot's support for binding username, password,
connection-timeout and read-timeout properties from `"spring.elasticsearch.rest"` namespace.

This commit enhances the `RestClientBuilderCustomizer` with support for customizing
`HttpAsyncClientBuilder` and `RequestConfig.Builder` by providing additional `customize`
methods that accept the aforementioned builders. Both new methods are optional as they have
no-op default implementations.

See gh-20994