[MERGE PG13][yugabyte#14506] Fix YbClearCurrentTransactionId()

Summary:
YbClearCurrentTransactionId() was introduced to handle temp tables created during concurrent
materialized view refreshes correctly.
PG13 no longer uses `CurrentTransactionState->transactionId` and `MyPgXact`. This diff changes the
code to use the PG13 equivalent of these fields/structures -
`CurrentTransactionState->fullTransactionId` and `MyProc->xid`.

Test Plan: Jenkins: skip

Reviewers: neil, smishra

Reviewed By: neil, smishra

Subscribers: yql

Differential Revision: https://phabricator.dev.yugabyte.com/D21721

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

@@ -6477,12 +6477,6 @@ void YBClearDdlHandles()
 
 void YbClearCurrentTransactionId()
 {
-#ifdef YB_TODO
-	/* YB_TODO(fizaa@yugabyte).
-	 * - PG13 data structures no longer have these fields.
-	 * - Need to reimplement clearing data accordingly to PG13.
-	 */
-	CurrentTransactionState->transactionId = InvalidTransactionId;
-	MyPgXact->xid = InvalidTransactionId;
-#endif
+	CurrentTransactionState->fullTransactionId = InvalidFullTransactionId;
+	MyProc->xid = InvalidTransactionId;
 }