fix: Fix Ordering issue in PBJ Comparable handling

[jsync-swirlds]

Note: This is not a fix. This is just changes needed to highlight the issue in CI and better describe what is failing. Fixing this issue will likely require a substantial effort.

• Added messages to test protobufs that fail in CI
• Adjusted getJavaFile to check existence and provide clearer error messages if a required file is missing
• Moved a few strings to constants in Common.java
• Removed a few unnecessary toString calls
• Fixed wildcard static import in Field.java

---

Additional minor adjustments

• Updated the Java language version to 21.
• Updated github CI config to use Java 21
• Moved the folder for the protobuf grammar file (Protobuf3.g4) to correct a package mismatch

[github-actions]

JUnit Test Report

     47 files  ±0       47 suites  ±0   2m 29s ⏱️ -34s
1 116 tests ±0  1 115 ✔️ ±0    1 💤 ±0  0 ❌ ±0 
6 971 runs  ±0  6 953 ✔️ ±0  18 💤 ±0  0 ❌ ±0 

Results for commit aff86a9. ± Comparison against base commit f6a4abc.

♻️ This comment has been updated with latest results.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.31% (target: -1.00%)	✅ 3.92%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (f6a4abc)	4263	1901	44.59%
Head commit (aff86a9)	4297 (+34)	1903 (+2)	44.29% (-0.31%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#189)	51	2	3.92%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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