Add "CR" as a stability level marker for release candidates.

[GeorgEchterling]

What?

Fixes #673.

Adds "CR" as a possible stability modifier for release candidates.

Why?

Hibernate uses "CR" ("Candidate Release") instead of "RC" to indicate release candidate versions. Previously, these were not properly detected.

How?

The extension method String.findStabilityLevel in the Version class now matches CR to StabilityLevel.ReleaseCandidate instead of null.

Testing?

I added some of Hibernate's CR versions to ReleaseCandidate.txt, which is used in VersionComparatorTest.testStabilityLevel. I then re-ran the check task in the plugins project.

[GeorgEchterling]

The failing test was "BundledDependenciesTest > Version keys should be up to date() FAILED". It passes locally, I don't know why it fails in CI.