Do not close old PRs that were manually updated

[fthomas]

This changes the logic for closing obsolete PRs such that only PRs which
were not manually updated are closed and the corresponding update branch
is deleted. If an obsolete update branch contains commits by authors
other than Scala Steward, only the "Superseded by" comment is posted to the
old PR.

Closes: #1871

[codecov]

Codecov Report

Merging #2254 (d7c0ea3) into master (d871194) will decrease coverage by 0.35%.
The diff coverage is 9.67%.

@@            Coverage Diff             @@
##           master    #2254      +/-   ##
==========================================
- Coverage   78.29%   77.93%   -0.36%     
==========================================
  Files         135      135              
  Lines        2271     2284      +13     
  Branches       50       58       +8     
==========================================
+ Hits         1778     1780       +2     
- Misses        493      504      +11     

Impacted Files	Coverage Δ
.../main/scala/org/scalasteward/core/git/Branch.scala	75.00% <0.00%> (-25.00%)	⬇️
...ala/org/scalasteward/core/nurture/NurtureAlg.scala	0.00% <0.00%> (ø)
...in/scala/org/scalasteward/core/git/GenGitAlg.scala	92.30% <50.00%> (+0.30%)	⬆️
...n/scala/org/scalasteward/core/git/FileGitAlg.scala	98.27% <66.66%> (+0.03%)	⬆️

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[fthomas]

This does not work if Scala Steward's commits are changed and force-pushed. An example is scalacenter/scalafix#1483.