Merge branch 'docs/add-gcp-sql-superuser-role' into 'master'

Add Google Cloud SQL superuser role

See merge request gitlab-org/gitlab!62512

doc/administration/postgresql/external.md

@@ -22,6 +22,7 @@ If you use a cloud-managed service, or provide your own PostgreSQL instance:
    roles to your `gitlab` user:
    - Amazon RDS requires the [`rds_superuser`](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Appendix.PostgreSQL.CommonDBATasks.html#Appendix.PostgreSQL.CommonDBATasks.Roles) role.
    - Azure Database for PostgreSQL requires the [`azure_pg_admin`](https://docs.microsoft.com/en-us/azure/postgresql/howto-create-users#how-to-create-additional-admin-users-in-azure-database-for-postgresql) role.
+   - Google Cloud SQL requires the [`cloudsqlsuperuser`](https://cloud.google.com/sql/docs/postgres/users#default-users) role.
 
    This is for the installation of extensions during installation and upgrades. As an alternative,
    [ensure the extensions are installed manually, and read about the problems that may arise during future GitLab upgrades](../../install/postgresql_extensions.md).