[#25336] YSQL: Disable check_is_install_user

Summary:
Vanilla PG expects one user during the upgrade, the bootstrap super user. This defaults to
`postgres`, but can be set with the `-U` argument to `initdb`.

In Yugabyte, the upgrade might be run by different Yugabyte users. Rather than try to capture every possible yugabyte user and rewriting the check to that, we can just ignore the user check. Since we control the creation of the new cluster, we don't have to be as vigilant against weird cases as Postgres does.

Fixes #25336
Jira: DB-14549

Test Plan:
```
./yb_build.sh release --cxx-test pg15_upgrade-test
```

Reviewers: hsunder

Reviewed By: hsunder

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D40726

src/postgres/src/bin/pg_upgrade/check.c

@@ -91,7 +91,6 @@ check_and_dump_old_cluster(bool live_check)
 	/* Extract a list of databases and tables from the old cluster */
 	get_db_and_rel_infos(&old_cluster);
 
-#ifdef YB_TODO
 	/* Enable these checks and other functions to initialize new node */
 	init_tablespaces();
 
@@ -101,13 +100,7 @@ check_and_dump_old_cluster(bool live_check)
 	/*
 	 * Check for various failure cases
 	 */
-	/*
-	 * YB: this check requires the following conditions:
-	 * 	1. The logged in user is 'postgres' (oid = 10)
-	 * 	2. New cluster does not have any users
-	 */
 	check_is_install_user(&old_cluster);
-#endif
 	check_proper_datallowconn(&old_cluster);
 	if (!is_yugabyte_enabled())
 		/* Yugabyte does not support prepared transactions, see #1125 */
@@ -208,7 +201,9 @@ check_new_cluster(void)
 	check_new_cluster_is_empty();
 	check_databases_are_compatible();
 
-	check_loadable_libraries();
+	if (!is_yugabyte_enabled())
+		/* YB: would fail with ERROR:  LOAD not supported yet. */
+		check_loadable_libraries();
 
 	switch (user_opts.transfer_mode)
 	{
@@ -222,10 +217,8 @@ check_new_cluster(void)
 			break;
 	}
 
-#ifdef YB_TODO
-	/* Investigate/implement this check */
 	check_is_install_user(&new_cluster);
-#endif
+
 	check_for_prepared_transactions(&new_cluster);
 
 	check_for_new_tablespace_dir(&new_cluster);
@@ -667,6 +660,14 @@ create_script_for_old_cluster_deletion(char **deletion_script_file_name)
 static void
 check_is_install_user(ClusterInfo *cluster)
 {
+	if (is_yugabyte_enabled())
+		/*
+		 * YB: Since there are a few different yugabyte users that might run the
+		 * upgrade, we disable this check rather than trying to modify it to
+		 * work for every possible YB case.
+		 */
+		return;
+
 	PGresult   *res;
 	PGconn	   *conn = connectToServer(cluster, "template1");
 

src/postgres/src/bin/pg_upgrade/pg_upgrade.c

@@ -159,10 +159,8 @@ main(int argc, char **argv)
 	if (!is_yugabyte_enabled())
 		start_postmaster(&new_cluster, true);
 
-#ifdef YB_TODO
-	/* Investigate relevant checks within check_new_cluster */
 	check_new_cluster();
-#endif
+
 	report_clusters_compatible();
 
 	pg_log(PG_REPORT,