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
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 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
