Bump baseline to Java 21

[uhafner]

Jenkins is moving to Java 21 baseline soon so we finally can make the same move as well.

[github-actions]

☀️   Quality Monitor

Tests

     Unit Tests: 100% successful (86 passed, 2 skipped)
   ⛔   Architecture Tests: 100% successful (17 passed, 2 skipped)

Code Coverage

   〰️   Line Coverage: 91% (64 missed lines)
   ➰   Branch Coverage: 90% (21 missed branches)

Mutation Coverage

     Mutation Coverage: 80% (80 survived mutations)
   💪   Test Strength: 85% (59 survived mutations in tested code)

Style

     CheckStyle: No warnings
     PMD: No warnings
   ☕   Java Compiler: No warnings

Bugs

     SpotBugs: No bugs
   🐛   Error Prone: No bugs

API Problems

   🚫   Revapi: No warnings

Vulnerabilities

   🛡️   OWASP Dependency Check: No vulnerabilities

Software Metrics

   🌀   Cyclomatic Complexity: 377 (total)
   💭   Cognitive Complexity: 174 (total)
   ➿   N-Path Complexity: 341 (total)
   📏   Lines of Code: 4284 (total)
   📝   Non Commenting Source Statements: 1261 (total)
   🔗   Class Cohesion: 71.43% (maximum)
   ⚖️   Weight of Class: 100.00% (maximum)

🚦 Quality Gates

Overall Status: ✅ SUCCESS

✅ Passed Gates

• ✅ Tests Success Rate: 100.00 >= 100.00
• ✅ Line Coverage: 91.00 >= 80.00
• ✅ Branch Coverage: 90.00 >= 80.00
• ✅ Potential Bugs: 0.00 <= 0.00
• ✅ Style Violations: 0.00 <= 0.00

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Created by Quality Monitor v3.5.0 (#e4cc310). More details are shown in the GitHub Checks Result.