Notify admins 15 days prior to license expiration

What does this MR do and why?

Related to #337250 (closed)

This is the second attempt at resolving the linked issue. My first attempt, !77608 (merged), had to be reverted because it was breaking reliable QA tests which wasn't seen until after the merge (see this thread for more info). Errors were raised for instances where the current license had no expiration date.

This MR adds back the logic which was reverted in the first commit. It also addresses the error caught by the reliable QA tests in the second commit.

Screenshots or screen recordings

Please see the first MR for this info.

How to set up and validate locally

Please see the first MR for this info.

MR acceptance checklist

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