Update core packaging to support a main branch

Problem/Motivation

When 11.0.0-alpha1 was tagged, 11.0.x-dev did not exist, breaking some assumptions in creating packaging jobs.

This was fixed with https://git.drupalcode.org/project/drupalorg/-/commit/ac28f1411b2a0d2fab...

This needs updating for 🌱 Use a 'main' branch as the development target Active

Proposed resolution

First, see if $last_packaged_commit_id is really needed by the subtree splitter for tagged releases. The dev release’s last packaged commit ID is diverged from the tag for security releases, and even in normal operations if dev happens to be pushed first. Avoiding all of the logic would be convenient.

If that does not work, then we have a reason to continue to require dev release nodes. (They are currently required for Composer, will no longer be required for testing & issues in GitLab.) Update the logic to use the main dev release when needed. This logic is shared with pift_ci_find_dynamic_release(), which needs to be removed from Project Issue File Test module anyway.