Bump process-compose version to 1.87.0 (#2777)

## Summary
- Update bundled process-compose from 1.64.1 to 1.87.0
- Remove unused `processComposeTargetVersion` constant
- Update flakeref test to use v1.87.0 (fixes test failure due to Go 1.19
dyld issue on macOS)

## Test plan
- [x] All tests pass (`go test ./...`)

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

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>