Refactor CI to use matrix strategy and add JDK 24 support

[xerial]

Summary

• Refactored CI workflow to use GitHub Actions matrix strategy for JDK testing
• Added JDK 24 to the test matrix
• Reduced CI configuration from ~100 lines to ~50 lines

Changes

• Replaced 4 individual test jobs (test_jdk8, test_jdk11, test_jdk17, test_jdk21) with a single matrix-based job
• Added JDK 24 to the test matrix, so tests now run on JDK 8, 11, 17, 21, and 24
• Each JDK version still runs both standard tests and universal buffer tests

Benefits

• Cleaner, more maintainable CI configuration
• Easy to add new JDK versions in the future (just add to the matrix array)
• Reduces duplication and potential for configuration drift
• All JDK versions use identical test steps

Test plan

• CI workflow syntax is valid
• All existing JDK versions (8, 11, 17, 21) continue to be tested
• New JDK 24 tests run successfully
• Both standard and universal buffer tests run for each JDK version

🤖 Generated with Claude Code