[#23661] YSQL: Change the default wal_level to logical

Summary:
wal_level is a GUC parameter that isn't applicable in YSQL since we don't use the PG WAL. There are a few logical replication clients which
validate the value of wal_level and ensure that it is set to 'logical' before proceeding with replication.

Those clients can fail when the value of wal_level isn't logical even though it isn't really applicable in YSQL. So for user experience, we are
setting the default value as 'logical' so that the number of user steps is reduced.
Jira: DB-12572

Test Plan:
Jenkins: test regex: .*ReplicationSlot.*

./yb_build.sh --java-test 'org.yb.pgsql.TestPgReplicationSlot#testDefaultWalLevel'

Reviewers: asrinivasan, skumar, sumukh.phalgaonkar

Reviewed By: sumukh.phalgaonkar

Subscribers: svc_phabricator, yql, ycdcxcluster

Differential Revision: https://phorge.dev.yugabyte.com/D37930

java/yb-pgsql/src/test/java/org/yb/pgsql/TestPgReplicationSlot.java

@@ -2893,4 +2893,12 @@ public void testDynamicTableWithReplicaIdentityChangeWithPgoutput() throws Excep
         + " plugin pgoutput. Consider using output plugin yboutput instead."));
     }
   }
+
+  @Test
+  public void testDefaultWalLevel() throws Exception {
+    try (Statement stmt = connection.createStatement()) {
+      Row row = getSingleRow(stmt, "SHOW wal_level");
+      assertEquals("logical", row.getString(0));
+    }
+  }
 }

src/postgres/src/backend/access/transam/xlog.c

@@ -101,7 +101,12 @@ char	   *wal_consistency_checking_string = NULL;
 bool	   *wal_consistency_checking = NULL;
 bool		log_checkpoints = false;
 int			sync_method = DEFAULT_SYNC_METHOD;
-int			wal_level = WAL_LEVEL_MINIMAL;
+/*
+ * YB NOTE: wal_level is not applicable to YB. So for user experience, we set
+ * the default to logical, so that any logical replication client doesn't throw
+ * any errors based on the value of the wal_level.
+ */
+int			wal_level = WAL_LEVEL_LOGICAL;
 int			CommitDelay = 0;	/* precommit delay in microseconds */
 int			CommitSiblings = 5; /* # concurrent xacts needed to sleep */
 int			wal_retrieve_retry_interval = 5000;

src/postgres/src/backend/utils/misc/guc.c

@@ -5417,7 +5417,12 @@ static struct config_enum ConfigureNamesEnum[] =
 			NULL
 		},
 		&wal_level,
-		WAL_LEVEL_REPLICA, wal_level_options,
+		/*
+		 * YB NOTE: wal_level is not applicable to YB. So for user experience,
+		 * we set the default to logical, so that any logical replication
+		 * client doesn't throw any errors based on the value of the wal_level.
+		 */
+		WAL_LEVEL_LOGICAL, wal_level_options,
 		NULL, NULL, NULL
 	},