scripts: take into account stable distro

As Mobian is going stable with bookworm, this commit adapts our repo management scripts to add the missing bits:

• deploy-pkg will deploy to bookworm-updates if the target distro (from the .changes file) is bookworm; this will also work in the future when trixie replaces bookworm as the stable distro
• migrate-pkg has a new -s option, migrating packages from bookworm-updates to bookworm; in this case, the packages are dropped from bookworm-updates
• remove-pkg now targets trixie and staging only as we won't be removing packages from the stable release

This is also the opportunity to drop the unstable repo (it has been a copy of staging for a while now, and is no longer useful) as well as the non-free component for both trixie and staging (so long, legacy code).