Docs update - Add extension plpgsql is required

doc/install/postgresql_extensions.md

@@ -15,6 +15,7 @@ The following extensions must be loaded into the GitLab database:
 |--------------|------------------------|
 | `pg_trgm`    | 8.6                    |
 | `btree_gist` | 13.1                   |
+| `plpgsql`    | 11.7                   |
 
 In order to install extensions, PostgreSQL requires the user to have superuser privileges.
 Typically, the GitLab database user is not a superuser. Therefore, regular database migrations