Elevate pre release semver

[ChristopherDavenport]

Found an error in SemVer when going from 0.20.0-M3 to 0.20.3 was labelled semver-patch which does not seem consistent with https://semver.org/

Hopefully this doesn't affect the system in another way I don't expect

[codecov]

Codecov Report

Merging #648 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #648   +/-   ##
=======================================
  Coverage   60.15%   60.15%           
=======================================
  Files          66       66           
  Lines         876      876           
  Branches       17       17           
=======================================
  Hits          527      527           
  Misses        349      349

Impacted Files	Coverage Δ
...ain/scala/org/scalasteward/core/model/SemVer.scala	100% <100%> (ø)	⬆️

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[fthomas]

LGTM. Thanks @ChristopherDavenport!

SemVer is currently only used for labelling, so this change is safe.