Auto-generate README with tag information

[malcyon]

It seems I've created a mess with having so many different image tags for the docker image (jdk-11, arm64, etc).

The idea here is to auto-generate the Readme using the format suggested in #191, so the releaser doesn't have to manually update each tag in the README.

Replace the static VERSION and VERSION_FORWARDER_PACKAGE files with a version.yaml file that the script can easily parse.

[malcyon]

My thought process on a possible path for improving the release automation:

• Create a new release.groovy Jenkins pipeline for handling the Docker release. It would handle updating the version.yml, regenerating the README, committing the changes, and adding a new tag. All the stuff we do manually right now.
• Have a second stage that checks out the new tag.
• Move the logic from jenkins.groovy that builds the Java 11 and ARM64 images into stage 2 of release.groovy.
• Add buildx commands to build graylog, graylog-enterprise, and forwarder into release.groovy. This should be easy since the current logic for the ARM64 and Java11 images already mimics all the weird regex tag stuff we do.
• Disable the Docker Hub automated builds.
• Now, all our official images would be multiplatform, which removes the need for all the extra arm64 tags. It'll be less confusing and easier to manage (See Image for arm64 architecture tag problem #175).
• Now that we're building the images on our own CI servers, adding in Content Trust (Enable content trust #23) in a later PR would be easier.

@bernd

[malcyon]

Alright, I am going to put the release pipeline stuff in a separate PR and restrict this to just the README changes.

[thll]

LGTM

[thll]

Looks good, but needs to be synchronized with the 4.2 branch now.