Add support for JDK 28 in Gradle workflow

[akarnokd]

Switch to oracle-actions/setup-java because they can run the very latest EA builds in a timely manner. Also make it runnable from the UI.

via Grok

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 5.102 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

.github/workflows/gradle_preview_jdks.yml: 5.102

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.61%. Comparing base (f611947) to head (32ecfa9).

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8251      +/-   ##
============================================
- Coverage     98.61%   98.61%   -0.01%     
+ Complexity     7129     7124       -5     
============================================
  Files           822      822              
  Lines         50690    50690              
  Branches       6892     6892              
============================================
- Hits          49990    49988       -2     
+ Misses          479      478       -1     
- Partials        221      224       +3     

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