Racket Version Compatibility

[6cdh]

Currently racket-langserver is compatible with Racket versions from 7.6 to latest 9.0. However, CI only tests versions 7.6 and 8.5. The supported version range is also not documented.

Proposed improvements:

1. Explicitly define and document the exact range of supported Racket versions.
2. Update CI to test all supported versions.
3. Consider dropping support for very old versions.
4. Introduce version-specific optimizations for newer Racket releases.
   For example, if version >= 9.0, use parallel thread, otherwise fallback to normal thread.

I've done a patch that uses parallel threads for check-syntax when running Racket 9.0+ and fallback to normal thread on old versions. And this change improves responsiveness a lot.