This commit fixes the missing pieces in our Maven Central publication
pipeline. Our first attempt at releasing with it showed a few problems:

* the promote task did not have the artifacts downladed with the
  artifactory repository
* we applied the wrong Sonatype credentials to the task
* the github changelog task would fail because of docker rate limiting
  since we were not using the right type of resource, which is
  configured with the proper caching mechanism

See gh-26654

Prior to this commit, the Spring Framework build would rely on
setting a custom Java HOME for building all sources and tests
with that JDK.

This approach is not flexible enough, since we would be testing
the source compatibility against a recent JDK, but not a common
case experienced by the community: compiling and running
application code with a recent JDK and the official, JDK8-based
Framework artifacts.
This method is also limiting our choice of JDKs to the ones
currently supported by Gradle itself.

This commit introduces the support of Gradle JVM Toolchains in
the Spring Framework build.

We can now select a specific JDK for compiling the main
SourceSets (Java, Groovy and Kotlin) and another one for
compiling and running the test SourceSets:

`./gradlew check -PmainToolChain=8 -PtestToolchain=15`

Gradle will automatically find the JDKs present on the host or
download one automcatically. You can find out about the ones
installed on your host using:

`./gradlew -q javaToolchains`

Finally, this commit also refactors the CI infrastructure to:

* only have a single CI image (with all the supported JDKs)
* use this new feature to compile with JDK8 but test it
against JDK11 and JDK15.

Closes gh-25787

This commit skips the Bintray-related tasks in our CI pipeline and
instead relies on Maven Central for publishing Spring Framework
artifacts.

This commit also updates the CI pipeline to sign the artifacts directly
with the `artifactory-resource`.

Closes gh-26654

This commit fixes the "optional-deployments" regex for ".zip" artifacts;
"\" do not need to be escaped in YML single-quoted strings.

Prior to this commit, the promote task in the CI release pipeline would
write the "concourse-release-scripts" CONSOLE logs to /dev/null; this
commit ensures that we can read those while promoting artifacts.

This commit configures the logging level to DEBUG for the concourse
release scripts tasks.

This is useful to get a clue on where those long-running tasks are at.

This commit adds new tasks to the release pipeline, generating release
notes automatically using the issues in the current milestone and
pushing that as the content of the GitHub release.

Closes gh-25922

This was introduced by mistake in 6b978cb5

Closes gh-25911

This release fixes a bug where Spring Boot specifics were baked in the
application itself.

Extract duplicate snippets as document anchors.

This also switches to the adoptopenjdk-produced binaries for better
consistency with other JDK versions in our CI.

This commit finishes the first step for migrating our complete build and
release pipeline to Concourse CI.

We're creating here a new "Release" jobs group in our pipeline.
We can now release Milestones from our pipeline with:

* the "stage-milestone" job, which creates and tags a new Milestone
version on the source repository, builds the artifacts and deploys them
in a staging repository and updates the version to the next SNAPSHOT.

* the "promote-milestone" fetches the build information from the staging
repository and promotes the released milestone in the official milestone
repository.

Interactions with the Artifactory and Bintray REST APIs are done using
the https://github.com/spring-io/concourse-release-scripts project.

Closes gh-22490

This new release has been updated with the version scheme changes.