Updated admin removal for package versions: using the flag on the action

[isoos]

• part of Package deletion admin action should just remove packages from public bucket #8730
• only changes the deletion of a version and not the package (yet, to be done in a follow-up PR)
• follows the same admin action pattern as the moderate actions
• reuses most of the update code with moderating and deleting a package version
• updated tests with only a few relevant bits, we won't need to duplicate everything, as the main code will be already tested with the moderation action
• does the actual deletion after 2 months

[isoos]

Note: also updated to merge/reuse the deletion of a package version.