Fix CDC decoder compilation and tuple lifetime on PG15 and PG16

On PostgreSQL 15 and 16, ReorderBufferChange data.tp.newtuple and oldtuple
are ReorderBufferTupleBuf pointers rather than HeapTuples, and their pointers
are not replaced during translation. Guard originalNewTuple and originalOldTuple
restoration and freeing with PG_VERSION_NUM >= PG_VERSION_17.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: d89d1f43-18ea-4405-bd1b-5c3129e391a9

Author: ihalatci

src/backend/distributed/cdc/cdc_decoder.c

@@ -268,8 +268,10 @@ TranslateAndPublishRelationForCDC(LogicalDecodingContext *ctx, ReorderBufferTXN
 	 * they were allocated from its own tuple context. Restoring the original
 	 * pointers after publishing keeps each allocator freeing only what it owns.
 	 */
+#if PG_VERSION_NUM >= PG_VERSION_17
 	HeapTuple originalNewTuple = change->data.tp.newtuple;
 	HeapTuple originalOldTuple = change->data.tp.oldtuple;
+#endif
 
 	PG_TRY();
 	{
@@ -294,6 +296,7 @@ TranslateAndPublishRelationForCDC(LogicalDecodingContext *ctx, ReorderBufferTXN
 		 * This also has to happen when the callback throws, because the reorder
 		 * buffer cleans the transaction up before the error propagates further.
 		 */
+#if PG_VERSION_NUM >= PG_VERSION_17
 		if (change->data.tp.newtuple != originalNewTuple &&
 			change->data.tp.newtuple != NULL)
 		{
@@ -307,6 +310,7 @@ TranslateAndPublishRelationForCDC(LogicalDecodingContext *ctx, ReorderBufferTXN
 			heap_freetuple(change->data.tp.oldtuple);
 			change->data.tp.oldtuple = originalOldTuple;
 		}
+#endif
 
 		RelationClose(targetRelation);
 	}