docs: add server release notes for 11.0.0

[DeepDiver1975]

ownCloud 11.0.0 was
published 2026-07-30 — the first major release of the 11.x line, with 11
security fixes across 47 changelog entries. This adds the
== Changes in 11.0.0 block to the server release notes.

Shape

Unlike the 10.16.x patch blocks, this one leads with Breaking Changes. A
major release is where admins most need the upgrade blockers up front rather
than buried in a flat change list:

• the PHP 8.3 floor (an instance on an older PHP will refuse to start),
• the group-admin feature now off by default behind allow_subadmins — existing
  assignments are ignored until an admin opts back in,
• the 2027-01-01 G1 signature sunset, which is the item most likely to bite
  anyone running third-party apps,
• the removed occ commands (integrity:sign-app/-core, db:convert-type)
  and legacy classes.

Then Security Fixes, Changes, and Notable Bugfixes.

#41634 is both a security fix and a breaking change. It's described in full
under Breaking Changes and cross-referenced from the Security Fixes list rather
than duplicated — the security list stays scannable and the count still adds up
to 11.

Verification

• All 11 Security entries in core's CHANGELOG.md for 11.0.0 are accounted
  for — diffed the PR-number sets, identical.
• All 38 PR/issue links checked against the API for existence and for
  pull-vs-issue kind (the changelog mixes both).
• Internal-only entries (CI, phan, JS test isolation) omitted: 36 of the 47
  entries are admin-relevant.

Note

The v11.0.0 GitHub release body deliberately links core's CHANGELOG.md
rather than server_release_notes.html#changes-in-11-0-0, because that anchor
didn't exist yet. Once this is merged and published, the release body can be
repointed at the docs (editing a release body touches no assets, so it's safe).