Upgrade dependencies and add comprehensive parameter validation

Major Changes:
- Upgrade Java baseline requirement from Java 8 to Java 21
- Update all Maven plugin dependencies to latest versions
- Update Jira REST client to 7.0.1 and Fugue to 6.1.2
- Add extensive parameter validation tests with edge case coverage

New Documentation:
- Add PARAMETER_VALIDATION.md documenting all parameter requirements
- Add unit test README explaining test structure and patterns

Testing Improvements:
- Add ParameterValidationTest with comprehensive coverage
- Add TestAbstractJiraMojo helper class for validation testing
- Test empty strings, null values, and authentication combinations
- Update AllJiraTests suite to include new tests

Dependencies:
- Keep Mockito at 4.x with comment explaining Java 11 baseline requirement
- Add maven-settings-builder for plexus-sec-dispatcher 4.x tests

POM Improvements:
- Improve XML formatting and readability
- Add explanatory comments for dependency decisions
- Consolidate version properties

Merge remote-tracking branch 'refs/remotes/origin/develop' into develop

Author: schrepfler