ci: Remove all CI except docs on EOL branches (v10)

[wolfgangwalther]

With #3113 in mind, we are going to keep some EOL-branches (currently v10 and v11) around for docs updates, but won't ever push bugfixes to them anymore. With renovate set up to update those older branches, too, this creates two problems:

• a considerable number of PRs to update CI dependencies to back-branches, too
• possibly failing builds, which can only be fixed by bigger changes to CI (e.g. FreeBSD builds/CI #2555)

While it's good to have working CI for docs and have renovate make sure the dependencies for this stay up2date - we don't need everything else. Thus, I propose to just remove all CI workflows except docs from the inactive, docs-only back-branches. This means:

• Do this PR
• Do the same against v11 (ci: Remove all CI except docs on EOL branches (v11) #3484)
• Do nothing against v12, because this is the last release branch, for which we might still want to release patch releases
• Going forward, whenever we release a new major, do the same on old major branches.

This also gives us much faster CI for those back branches and takes fewer resources for nothing.

This would also supersede #3459, at least for the v10 and v11 branches. We'd need to look into v12 again.

[wolfgangwalther]

This would also supersede #3459, at least for the v10 and v11 branches. We'd need to look into v12 again.

v12 doesn't have the code checking for rel- branches anymore, so doesn't need to be adjusted.