Retire the bridge to the old dev channel release

Scribe #3884. The dev channel's release tag moved from `dev` to `dev-rolling`
on 2026-09-10. Since then, the manifest job has also written latest.json to the
old `dev` release, so that desktop apps installed before the move could update
across. The operator has had two desktop installs and is fine reinstalling,
so the bridge goes. The old release and tag are deleted next, through the forge.

- desktop.yml: the BRIDGE_TAG=dev export is removed.
- write-manifest.sh: the TEMPORARY bridge block is removed.
- ci-requirements.md: the "Transitional" paragraph becomes a note that the tag
  is gone, and that an app installed before 2026-09-10 reinstalls with
  install.sh.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.forgejo/workflows/desktop.yml

@@ -517,13 +517,6 @@ jobs:
             main) RELEASE_TAG="$(sh packaging/channel-tag.sh stable)"
                   export RELEASE_NOTES="Stable build from ${GITHUB_SHA}" ;;
             *)    RELEASE_TAG="$(sh packaging/channel-tag.sh dev)"
-                  # TEMPORARY one-shot bridge (Scribe #2184). Desktop apps installed
-                  # before the rename have .../download/dev/latest.json compiled in,
-                  # so the manifest is also written to the old `dev` release; its
-                  # URLs name dev-rolling assets, so those apps update once into a
-                  # build that reads the new tag. Delete this line together with the
-                  # old `dev` release and tag.
-                  export BRIDGE_TAG=dev
                   export RELEASE_NOTES="Development build from ${GITHUB_SHA}" ;;
           esac
           # Assigned bare above so a failing channel-tag.sh fails this step;