genericcache: investigate replacing with Otter v2

[`genericcache.Cache`](https://github.com/cockroachdb/pebble/blob/master/internal/genericcache/cache.go#L14-L20) implements a sharded cache using CLOCK-Pro. We should investigate whether this cache can be replaced with [Otter v2](https://github.com/maypok86/otter?tab=readme-ov-file) which is optimized for read-heavy workloads which is what we expect from the [`FileCache`](https://github.com/cockroachdb/pebble/blob/master/file_cache.go#L407-L409) used to cache open file handles.

Jira issue: PEBBLE-538

Start by reading internal/genericcache/cache.go and the FileCache implementation in file_cache.go, then review the linked Otter v2 documentation. Compare whether Otter v2 is suitable for the read-heavy open-file-handle workload and document a clear replacement recommendation.

Written by the indexing model from the issue text.