Release: deprecate non-GitHub Qt 5 packages

[pbek]

Summary

Stop publishing Qt 5 packages through the local Bitwarden-driven release process. Keep the Qt 5 artifacts that are built directly by the GitHub Actions release workflow for now.

Scope

• Remove the legacy Qt 5 Ubuntu Launchpad PPA job from the release orchestration.
• Remove the legacy Qt 5 OBS desktop job from the release orchestration.
• Remove Qt 5 and CDBS-only dependencies and checks from the release container.
• Keep the Qt 6 Launchpad PPA, Qt 6 OBS desktop-cmake3, Launchpad Snap, AUR, Gentoo, and Slackware deployments.
• Keep the Qt 5 Snap and macOS artifacts currently produced directly by GitHub Actions.
• Keep Qt 5 source compatibility and CI coverage until a separate decision removes them.
• Update installation and internal retirement documentation to describe the remaining support boundary.

Rationale

The supported external package targets now have Qt 6 deployment paths. Continuing to publish the legacy Launchpad and OBS Qt 5 packages duplicates release work and keeps the local release container tied to the old qmake/CDBS packaging stack. Ubuntu 20.04 Focal cannot use the current Qt 6 PPA dependencies from its standard archive; affected users can remain on the final Qt 5 package or use another distribution format.

Acceptance criteria

• build-systems/build-with-bitwarden.sh no longer reaches a Qt 5 deployment through build-all.sh or releaser.kdl.
• The local release container no longer installs Qt 5/CDBS solely for retired deployment jobs.
• Active local release scripts explicitly use Qt 6 translation tools.
• GitHub Actions Qt 5 release artifacts remain unchanged.
• The changelog references this issue.