Pin gluetun in values too, so Renovate proposes it there

The chart bundles nine independently versioned upstreams, and eight of them
were already pinned in values.yaml while only gluetun took its tag from
appVersion. That split had no upside: it made gluetun the one image Renovate
could not reach through the built-in helm-values manager, and it implied a
"main app" the stack does not have.

gluetun now carries gluetun.image.repository/tag like every other image, and
the StatefulSet renders it from there. No tag comes from appVersion any more.
Renovate picks all nine up through helm-values - the manager the repo already
relies on for sidecar images - and ci/renovate-bump-chart.sh bumps the chart
version with each update.

Consequences, all documented where they bite:
- renovate.json5 deliberately gets no app-version block for this chart. One
  would either duplicate the helm-values updates or rewrite an appVersion that
  selects no image. The absence is commented so it does not read as an
  oversight.
- freesailarr leaves the change-app-version.yml choice list. That workflow
  rewrites appVersion and republishes, which for this chart would change no
  deployed image at all.
- appVersion becomes a hand-maintained marker for the gluetun line. Since
  renovate-bump-chart.sh bumps the patch level on an unchanged appVersion, a
  gluetun update has to be paired with a hand edit of appVersion, or the +up
  suffix stops describing what the chart deploys. Noted in Chart.yaml,
  values.yaml and README.

.github/workflows/change-app-version.yml

@@ -35,7 +35,11 @@ on:
           - minecraft-router
           - minecraft-server
           - rss-bridge
-          - freesailarr
+          # freesailarr is deliberately absent: none of its nine images takes
+          # its tag from appVersion (they are all pinned in its values.yaml),
+          # so this workflow would rewrite appVersion and republish the chart
+          # without changing a single deployed image. Update its images through
+          # a Renovate PR, and its appVersion by hand.
   workflow_call:
     inputs:
       app-version: