Rename "schema cache" to "internal cache"

[steve-chavez]

Since the introduction of Multiple Schemas we no longer have a single schema cache, so the term "schema cache" is not apt anymore.

Also, seems it can confuse users to create a pg schema named "cache" https://github.com/PostgREST/postgrest/discussions/2172#discussioncomment-2202891

So, I think it'd be better to change the name to "internal cache".