Previously, Spring Boot's modules published Gradle Module Metadata
(GMM) the declared a platform dependency on spring-boot-dependencies.
This provided versions for each module's own dependencies but also had
they unwanted side-effect of pulling in spring-boot-dependencies
constraints which would influence the version of other dependencies
declared in the same configuration. This was undesirable as users
should be able to opt in to this level of dependency management, either
by using the dependency management plugin or by using Gradle's built-in
support via a platform dependency on spring-boot-dependencies.

This commit reworks how Spring Boot's build uses
spring-boot-dependencies and spring-boot-parent to provide its own
dependency management. Configurations that aren't seen by consumers are
configured to extend a dependencyManagement configuration that has an
enforced platform dependency on spring-boot-parent. This enforces
spring-boot-parent's version constraints on Spring Boot's build without
making them visible to consumers. To ensure that the versions that
Spring Boot has been built against are visible to consumers, the
Maven publication that produces pom files and GMM for the published
modules is configured to use the resolved versions from the module's
runtime classpath.

Fixes gh-21911

See gh-21938

See gh-21937

This commit adds a runImage property to the Maven plugin build-image
goal and the Gradle bootBuildImage task. The property allows the user
to override the run image reference provided in the builder metadata
with an alternate run image. The runImage property can be specified
in the build file or on the command line.

Fixes gh-21534

Update the releasescript to ensure that a distribution has been started
before starting it again. This functionality was accidentally removed
during the updates for gh-21474.

See gh-21474

Prior to this commit, the welcome page support implemented in gh-9785
would override existing index views in both annotation and functional
variants.

This comes from the fact that the feature was implemented as a
`RouterFunction` configured in the main `RouterFunctionMapping` bean.
Due to ordering issues between mappings, this would override existing
application mappings in some cases.

This commit ensures that the welcome page `RouterFunction` is
contributed to the context in its own handler mapping, ordered after the
application ones.

Fixes gh-21909

Closes gh-21910

Closes gh-21916

* pr/21906:
  Update copyright year of changed files
  Fix wrong property reference to keys-to-sanitize in Javadoc

Closes gh-21906

See gh-21906

See gh-21906

Closes gh-21908

Update `AvailabilityChangeEvent` to be a `PayloadEvent` and ensure
that the `getResolvableType` method returns a generic compatible
result.

Prior to this commit, a ClassCastExeption would be thrown if the
following event listener was declared:

  @EventListener
  void onEvent(AvailabilityChangeEvent<ReadinessState> event) {
    ...
  }

Closes gh-21898

Cherry-pick release script updates from 2.3.x

See gh-21474

See gh-21474

Improve logging and fix a few issue with the release script.

See gh-21474

Update the release tooling to check for bintray published artifacts
using SHA256 digests and to also check before attempting a promote.

See gh-21474

Reinstate test now that we are on a reactor-netty releases that fixes
reactor/reactor-netty issue 1093.

Closes gh-21437

Further attempt to fix Windows file issues.

See gh-21575

Update `BuildRequest` to also allow digest based references.

Closes gh-21879

Refactor the internals of `PropertiesLauncher` so that opened jar files
are tracked and can be closed after a test completes.

See gh-21575

Update Ant smoke test to explicitly use Java 8.

* pr/21872:
  Fix deprecation in NettyRSocketServerFactoryTests

Closes gh-21872

See gh-21872

* pr/21793:
  Link complete Slack message to build log

Closes gh-21793

See gh-21793

This commit changes the NamedPipeSocket used for communication with
a local Docker daemon to use a non-blocking AsynchronousByteChannel
instead of a blocking RandomAccessFile, modeled after a similar
change to the docker-java project. This eliminates the potential for
a blocking call to hang indefinitely.

Fixes gh-21672

Pick up the workaround for the AsciidoctorJ bug that prevents the
configprops macro's attributes from being parsed. The lack of parsing
meant that the envvar format was being lost and properties were being
written in the canonical form instead.

Closes gh-21794