IONOS(mail): update submodule to v5.12.3 IONOS port (NC33, NSW-1070)

[printminion-co]

Moves apps-external/mail from v5.5.11 + IONOS to upstream nextcloud/mail v5.12.3 + IONOS, so the whole build and the resulting artifact can be checked before the ncw-mail stack is merged.

Important

Draft, do not merge yet. The gitlink points at 9fdc208, the head of the unmerged ncw-mail stack:
IONOS-Productivity/ncw-mail#61 → #62 → #63 (base ionos-dev-5.12).
Once the stack is merged, I'll replace this pointer with the release tag v5.12.3-<sha7> and mark the PR ready.

What changes for mail

• Upstream jump: v5.5.11 → v5.12.3. info.xml declares NC 32–35. The upgrade runs the upstream migrations from 5.5.11 on.
• IONOS delta: all 148 IONOS commits on the live ionos-dev are replayed (NSW-782 from draft PR IONOS(collectives): add submodule v3.2.0 (22f7609) #55 is not included). Six that are obsolete or already upstream were dropped; the ncw-mail PR descriptions list every one.
• New: the allow_new_mail_aliases admin toggle, doc/ionos-admin.md, and regenerated IONOS translations. The regeneration also fixes the plural translations that Phrase had broken on ionos-dev.
• Build: no IONOS/Makefile change needed.
  • Node goes from ^22 to ^24 (and npm from ^10.5 to ^11.3). This repo's package.json already declares ^24, which the build job uses.
  • mail now runs a patch-package postinstall during npm ci.
  • The private ionos-mail-configuration-api-client composer package is still required, same as before.

How to check

1. Watch the build-artifact run on this PR. In particular, the build_mail_app matrix job must pass.
2. Spin up the artifact with .dev/container/check_release.sh pr-387 ncw.
3. Check:
   • occ upgrade from an existing 5.5.11 instance completes.
   • occ mail:provider:list and occ mail:provider:status ionos work.
   • Admin settings → Mail provider accounts lists mailboxes.
   • With allow_new_mail_aliases=no, the "Add alias" button is hidden.
   • DE/FR strings are translated.

Locally I already ran the 5.5.11 → 5.12.3 upgrade on a throwaway ionos-dev-v33 install with SQLite. It passed, with the production-style mixed allow_new_mail_aliases=no intact afterwards and alias creation blocked.

Unrelated to the open fork-app sweep #381, which doesn't touch mail.

Jira: NSW-1070