Add multi-JDK CI matrix to sheetz-examples

[chitralabs]

Improvement: Test Examples on Java 17 and 21

The sheetz-examples CI currently only tests on one Java version.
We should verify examples run correctly on Java 11, 17, and 21.

Task:
Update .github/workflows/ci.yml in sheetz-examples to add:

matrix:
  java: ['11', '17', '21']
  os: [ubuntu-latest, windows-latest]

Difficulty: Very Easy — just YAML changes
Time estimate: ~15 minutes

[mustafaguler3]

Hi 👋 I’d like to work on this issue. Can I take it?