Only validate the first schema in `db-schemas`

[steve-chavez]

Problem

Adding schemas in db-schemas and then dropping them database side makes postgREST inoperative, it won't serve any requests.

After much discussion on #4364 and #5042, it seems we're stuck with db-schemas for some cases.

Solution

I suggest only validating the first schema in db-schemas, since it will be picked as the "default schema". The rest don't have to be validated.

I also believe this goes in line with recent changes about making the schema cache "best effort" (#4869)..

[steve-chavez]

Closing this as I think it's better to focus on the proposals in #5042. It'd be great if we can get rid of db-schemas.