* Fix mvnup tool issues apache#7934-apache#7938

This commit addresses five critical issues with the Maven upgrade tool (mvnup):

**Issue apache#7934: External parent preservation**
- Fixed InferenceStrategy to detect external parents vs reactor parents
- External parents now preserve required groupId, artifactId, and version elements
- Prevents 'parent.groupId is missing' errors when using META-POMs

**Issue apache#7935: .mvn directory creation for all model versions**
- Modified AbstractUpgradeGoal to create .mvn directory for both 4.0.0 and 4.1.0
- Helps Maven find project root and avoids root directory warnings

**Issue apache#7936: Downgrade error handling**
- Changed ModelUpgradeStrategy to fail with errors instead of warnings for invalid downgrades
- Tool now properly exits with error code when attempting 4.1.0 → 4.0.0 downgrade

**Issue apache#7937: Unicode icon fallback**
- Enhanced UpgradeContext to detect Unicode support using terminal.stdoutEncoding()
- Falls back to ASCII characters ([OK], [ERROR], etc.) when Unicode not supported
- Improves compatibility with Windows CMD/PowerShell and other terminals

**Issue apache#7938: Child version removal in 4.1.0**
- Enhanced inference logic to properly remove matching child versions in subprojects
- Ensures child project versions are removed when they match parent version

**Testing:**
- Updated existing tests to reflect correct behavior
- Added new tests for downgrade handling and Unicode detection
- All mvnup tests passing with comprehensive coverage

Fixes apache#7934, apache#7935, apache#7936, apache#7937, apache#7938

* Improve parent reactor detection logic

- Replace heuristic-based approach with direct GAV matching against pomMap
- More accurate detection of external vs reactor parents
- Added comprehensive test coverage for both scenarios
- Suggested by code review feedback

* Refactor to use GAVUtils.extractGAVWithParentResolution

- Replace manual GAV extraction with existing GAVUtils method
- More robust handling of parent resolution and inheritance
- Leverages existing tested utility for GAV extraction
- Cleaner and more maintainable code

* Address review feedback: Implement ConsoleIcon enum with charset detection

This commit addresses all review feedback from @elharo and @Bukama:

**Icon Management Improvements:**
- Created ConsoleIcon enum with Unicode/ASCII fallback pairs
- Moved all icon logic into the enum for consistency and reusability
- Implemented ConsoleIcon.getIcon(Terminal) for clean integration

**Unicode Detection Enhancements:**
- Use Charset.newEncoder().canEncode(char) for accurate character testing
- Use terminal.encoding() instead of deprecated stdoutEncoding()
- Simplified fallback to Charset.defaultCharset()
- Removed complex heuristics and exception handling

**Code Quality Improvements:**
- UpgradeContext methods are now clean one-liners
- Better separation of concerns between icon rendering and context management
- Comprehensive test coverage for all scenarios
- Future-proof design for adding new icons

**Review Comments Addressed:**
- @elharo: 'maybe this should be a field that can be shared?' → Icons now shared via enum
- @Bukama: 'maybe an enum with icon and fallback text?' → Implemented with Terminal integration
- @elharo: 'There are others that also support Unicode, e.g. UCS-2., UCS-4' → Now tests actual encoding capability
- @elharo: 'Please be specific about exceptions' → No longer needed with simplified approach
- @elharo: 'toLowerCase(Locale.ENGLISH)' → No longer needed with canEncode() approach

**Technical Benefits:**
- More accurate Unicode detection per character
- Works with any charset automatically
- Cleaner and more maintainable code
- Better test coverage and reliability

(cherry picked from commit adf2c49)