Discussion on HLS cache management.

HLS cache limits have been a concern for some time. we are always close to our 10GB limit. This made me dig deeper into our cache eviction policies. Below is my documentation for it and blockers that needs more opinions.

some ways to improve cache utilization :
1. Evict cache entries for PRs that are merged / closed ( attempted [here ](https://github.com/haskell/haskell-language-server/blob/master/.github/workflows/clear-cache.yaml))
2. Evict old cache entries when new cache entries are created.

These are only some of the ideas, welcoming more ways ;

Blockers :
- PR #4868 and PR #4865 introduced cache cleaning when a PR is closed / merged but currently Github workflows do not have access to write.

- To begin working on any of the cache eviction we first need to give write access to our workflows.
- will this / could this create any unwanted side effects later is a point to discuss.