Update CONTRIBUTING.md for freshness

Contributing doc is a bit stale and should be brought up-to-date.

https://github.com/GoogleCloudPlatform/cloud-sql-jdbc-socket-factory/blob/main/CONTRIBUTING.md

Potential changes:

- Update triage priorities
- How to lint?
- Verify running tests section
- How to run a single test and not all tests?