Add regex filtering for PR labels

[ybasket]

Filter labels Scala Steward adds to PRs it creates by a regex to avoid visual clutter or conflicts with existing label conventions. Follows up on discussion that happened in #2523.

[codecov[bot]]

Codecov Report

Merging #2636 (805df46) into main (fcd5ea3) will increase coverage by 0.00%. The diff coverage is 88.88%.

@@           Coverage Diff           @@
##             main    #2636   +/-   ##
=======================================
  Coverage   81.32%   81.33%           
=======================================
  Files         146      146           
  Lines        2592     2598    +6     
  Branches       43       40    -3     
=======================================
+ Hits         2108     2113    +5     
- Misses        484      485    +1     

Impacted Files	Coverage Δ
...ala/org/scalasteward/core/nurture/NurtureAlg.scala	0.00% <0.00%> (ø)
...lasteward/core/repoconfig/PullRequestsConfig.scala	91.66% <100.00%> (+4.16%)	:arrow_up:
...calasteward/core/vcs/data/NewPullRequestData.scala	100.00% <100.00%> (ø)

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