Pair stack releases with maintenance branches

[mdelapenya]

BTW, starting with Metricbeat 7.8.0, we will no longer need to specify the metricsets when xpack.enabled: true is set: elastic/beats#17609. Since this PR here is targeting master, I think we should remove metricset. When we backport to 7.7 and 6.8 (we will need these branches in this repo, BTW, similar to how https://github.com/elastic/elastic-stack-testing has branches corresponding to product repos), we can add them back.

That would be great. Not something we need to do just yet, but just a thought for the future when we start seeing differences between branches (like the one I mentioned above).

Originally posted by @ycombinator in #113

[mdelapenya]

We will be able to remove the version parameters in the Jenkinsfile, as it will be controlled by the maintenance branches

[mdelapenya]

Closing, as the maintenance branches were created