sql: allow ALTER DATABASE PRIMARY REGION to work on system tenants

Previously, on a multi-region setup the system database could not be modified to be multi-region and it was blocked from being made multi-region aware. To address this, we are now allowing ALTER DATABASE
PRIMARY REGION to work on system tenants.

Fixes: #63365
Epic: CRDB-33032

Release note (sql change): Previously, we added support for settings reegion on the system database, which was limited to tenants only. We lifted this limitation to allow ALTER DATABASE PRIMARY REGION to work on system tenants.
To support preview status, we created a cluster setting called sql.multiregion.preview_multiregion_system_database that will give users the option to set up their system database as multi-region for Cockroach dedicated
(this cluster setting is not enabled by default). Note that after adding non-primary regions, we recommend that users do a rolling restart to propogate region information.

pkg/ccl/logictestccl/testdata/logic_test/multi_region

@@ -544,7 +544,7 @@ ALTER DATABASE system PRIMARY REGION "ap-southeast-2"
 # are in the system tenant. Hence the lack of error code checking and the
 # regex matching both possible outcomes.
 skipif config multiregion-9node-3region-3azs-tenant
-statement error user testuser may not modify the system database|modifying the regions of system database is not supported
+statement error user testuser may not modify the system database|Modifying the regions of system database is not supported. Set up your system database as multi-region using the cluster setting `sql.multiregion.preview_multiregion_system_database.enabled` https://www.cockroachlabs.com/docs/stable/cluster-settings
 ALTER DATABASE system PRIMARY REGION "ap-southeast-2"
 
 user root