pg_upgrade: check for removed gucs

[divyeshddv]

Gucs/settings can be applied to certain databases or roles and are stored in pg_db_role_settings Upgrade would try to restore these settings on target cluster Some of these settings are removed in newer version and will cause upgrade to fail. This happens during metadata restore on the target cluster because pg_restore will error trying to apply these settings, and would fail since these settings are not present anymore. This is not ideal as we could be many hours into upgrade before a removed guc causes pg_upgrade to fail This check calls a support function on the source cluster to check if any such removed gucs are applied by the user on any of the dbs/roles.

7X check PR: https://github.com/greenplum-db/gpdb/pull/17373 Gpupgrade test PR: https://github.com/greenplum-db/gpupgrade/pull/912