ci(release): stop building Sparkle deltas

[datlechin]

Why

Delta updates are not wanted. They took the release job from about a minute to eight, and the 0.81.0 deltas were 14 to 18 MB against a 20 MB full archive.

Change

• The release job no longer fetches earlier archives or builds deltas. sign-and-appcast.sh signs one tar.xz per architecture, as before ci(release): publish Sparkle deltas for the last five releases #3362, and the job timeout goes back to 10 minutes.
• Removed fetch-delta-bases.sh, name-deltas.py and their tests, plus the delta handling in appcast_feed.py and merge-appcast.py.
• appcast.xml drops the 0.81.0 delta enclosures, so every install downloads the full tar.xz.
• The 0.81.0 CHANGELOG entry gives the real sizes.

Verified

sign-and-appcast.sh 0.81.0 run on the published 0.81.0 archives, re-keyed to a throwaway key: one signed item per architecture, no delta, the critical flag kept, merge validated. scripts/ci/test_*.py, shellcheck and actionlint pass. test_merge_appcast.py lost its five delta cases.