[yugabyte#23799] test: Fixed PgTableSizeTest.SharedTableSize test for pg15

Summary:
The test `PgTableSizeTest.SharedTableSize` was checking the size of
the shared tables: `pg_auth_members` & `pg_pltemplate`.
For PG15 the table `pg_pltemplate` (supported till PG12 only) is replaced
by `pg_authid`, because the table is not available in PG15.

The fix is implemented in the `master` branch to have it sync-ed with
the `pg15` branch.
Jira: DB-12701

Test Plan: ./yb_build.sh --cxx-test pgwrapper_pg_table_size-test --gtest_filter PgTableSizeTest.SharedTableSize

Reviewers: mihnea, jason, sushil.kumar

Reviewed By: jason

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D37718

Author: OlegLoginov

src/yb/yql/pgwrapper/pg_table_size-test.cc

@@ -226,7 +226,7 @@ TEST_F(PgTableSizeTest, SharedTableSize) {
   ASSERT_OK(cluster_->CompactTablets());
 
   ASSERT_OK(VerifyTableSize(
-      cluster_.get(), &test_conn, "T", "pg_pltemplate", true /* relation_is_catalog_table */));
+      cluster_.get(), &test_conn, "T", "pg_authid", true /* relation_is_catalog_table */));
   ASSERT_OK(VerifyTableSize(
       cluster_.get(), &test_conn, "T", "pg_auth_members", true /* relation_is_catalog_table */));
 }