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 https://github.com/Graylog2/graylog-docker/pull/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 #175).
• Now that we're building the images on our own CI servers, adding in Content Trust (https://github.com/Graylog2/graylog-docker/issues/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.