Update cockroachdb provider to specific string instead of postgres

[jharrell]

Should now read cockroachdb instead of postgresql

I think this is already the case in our docs, but we should go through and make sure we are consistent.

[janpio]

Should indeed not be documented in the old way we are now removing.

Might be useful to make that explicit in the CockroachDB docs though, and also explain to users why (different native types I think mainly).