squawk: Specify PostgreSQL version in config (#28094)

This PR adds the PostgreSQL version to the squawk config, see
https://squawkhq.com/docs/cli#specifying-postgres-version for reference.

The specified version matches the PostgreSQL version in the compose-file


43cb925a59/compose.yml (L3)

and prevents false positives like
https://github.com/zed-industries/zed/pull/28090#issuecomment-2778871346
from happening (tested it locally with that commit).

Release Notes:

- N/A

script/lib/squawk.toml

@@ -8,3 +8,4 @@ excluded_rules = [
     "prefer-big-int",
     "prefer-bigint-over-int",
 ]
+pg_version = "15.0"