fix: retire_old_licenses ignores test plans

[iloveagent57]

Description

I'd like to ignore test plans from license retirement, so that in environments with plans that have large numbers of assigned licenses for load/scale-testing, we don't OOM when retiring assigned licenses that have exceeded their purge duration. This is a stop-gap solution - what we really need are changes to

1. Make this mgmt command work in smaller chunks, for scalability, and
2. A way to quickly/cleanly/efficiently delete test plans in certain environments.

Testing considerations

• Include instructions for any required manual tests, and any manual testing that has
  already been performed.
• Include unit and a11y tests as appropriate
• Consider performance issues.
• Check that Database migrations are backwards-compatible

Post-review

Squash commits into discrete sets of changes

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