ci: add target bytecode version matrix to build-and-test

Run build-and-test on all 3 target bytecode versions (1.5, 1.6, 1.7)
to ensure JOPA works correctly across all supported output formats.

Combined with the existing debug/release variant matrix, this gives
6 total configurations for comprehensive coverage.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

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wip: java 7, more tests

wip: java 7

wip: java 7

cleanups