ci/docs: drop current branch references after rename to rolling

Cleanup PR following the current→rolling default-branch rename. Drops
the now-stale current branch from workflow triggers, flips remaining
defaults, and updates documentation/templates.

Workflows:
- pr-mirror-repo-sync.yml: branches narrowed to [rolling]; workflow_dispatch
  default and choice list now rolling-only; sync_branch fallback flipped
  from 'current' to 'rolling'.
- check-open-prs-conflict.yml: dropped current from push.branches.
- update-version-tags.yml: dropped current from push.branches and from
  the case statement (now bare 'rolling) TAG=rolling').
- submodules.yml: renamed update_current job, branch
  update-dependencies-current, and the related commit/title strings to
  rolling.
- update-translations.yml: PR branch update-translations-current renamed
  to update-translations-rolling.
- ai-validation.yml: prompt example updated.

Docs/templates:
- docs/conf.py: READTHEDOCS_GIT_IDENTIFIER fallback flipped to 'rolling';
  comment block updated to describe the rolling branch (the user-facing
  "(current)" suffix is release-channel terminology and remains).
- README.md: branching-scheme section now describes rolling as the
  default branch, with a note about the rename date.
- .github/PULL_REQUEST_TEMPLATE.md: CONTRIBUTING link points at
  blob/rolling.

External @current refs to vyos/.github and vyos/vyos-cla-signatures are
left untouched — they reference those repos' default branches and are
unaffected by this repo's rename.

🤖 Generated by [robots](https://vyos.io)

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