The latest release of all `pg_dump` major versions, going back to 13,
started inserting `\restrict` `\unrestrict` keywords into dumps. This
currently breaks sqlc in `gen/dump` and our check migration script. Full
details of the postgres change are available here:
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=575f54d4c

To fix, we'll always use the `pg_dump` in our postgres 13.21 docker
image for schema dumps, instead of what's on the runner/local machine.

Coder doesn't restore from postgres dumps, so we're not vulnerable to
attacks that would be patched by the latest postgres version.
Regardless, we'll unpin ASAP.

Once sqlc is updated to handle these keywords, we need to start
stripping them when comparing the schema in the migration check script,
and then we can unpin the pg_dump version. This is being tracked at
coder/internal#965

(cherry picked from commit 1b4ce09)