Automate updates of LTS version

[timja]

What feature do you want to see added?

#1097 introduces managing of the LTS version as configuration rather than automatically figuring it out. This allows us to get more deterministic results and review when breakages occur.

#1097 (comment) shows where needs to be automatically updated

The easiest way is likely with updatecli, see:
https://github.com/jenkins-infra/jenkins-infra/blob/production/updatecli/weekly.d/jenkins-lts.yaml

and could be run with a GitHub action, see https://github.com/jenkinsci/bom/blob/master/.github/workflows/action-updatecli.yaml

Upstream changes

No response