Make Groovy major version parsing more robust for groovy-eclipse-compiler

[AndreasTu]

See #2282 for more details.

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.12%. Comparing base (8110e0f) to head (d68fdec).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
.../org/spockframework/runtime/GroovyRuntimeUtil.java	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2285      +/-   ##
============================================
- Coverage     82.13%   82.12%   -0.02%     
  Complexity     4794     4794              
============================================
  Files           469      469              
  Lines         14958    14963       +5     
  Branches       1888     1889       +1     
============================================
+ Hits          12286    12288       +2     
- Misses         1984     1986       +2     
- Partials        688      689       +1     

Files with missing lines	Coverage Δ
.../org/spockframework/runtime/GroovyRuntimeUtil.java	80.80% <57.14%> (-2.18%)	⬇️

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[AndreasTu]

Is already fixed by the Eclipse-Groovy-Compiler, see groovy/groovy-eclipse#1648
So we do not need this change here.