PR enforcer workflow: use regex to filter labels starting with [Type]

[priethor]

What?

Updates the PR label enforcement automation to simply require one and only one label that starts with [Type].

Why?

Maintaining the list with individual labels is not only inconvenient, but indicates an inconsistency in the labels: any type-related label should follow the same prefix!

How?

By using a regular expression.

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[desrosj]

This seems good, @priethor!

I was trying to think of any scenarios where the regex would unintentionally match something incorrectly. The only thing I could think of is if there were old, retired [Type] labels that are on a PR. Perhaps a new label is created to replace the previous one. But I don't think that's a blocker here. We can figure that out if it actually comes up.