GHA: Require changelog label on PRs

[hugovk]

Require one of the "changelog: X" labels on PRs.

https://github.com/mheap/github-action-required-labels

[codecov]

Codecov Report

Merging #100 (102d489) into main (686cba5) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #100   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           13        13           
=========================================
  Hits            13        13           

Flag	Coverage Δ
GHA_Ubuntu	100.00% <ø> (ø)
GHA_macOS	100.00% <ø> (ø)

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