rgw: fix the version in DOCODE_START() of RGWZoneGroupPlacementTier

This commit fixes an undetected merge conflict between PRs ceph#61745
and ceph#60159. The dencoding problem has been introduced very recently,
it is straightforward and causes failures of the make check bot
everywhere, therefore -- if no objections -- I want to merge this
patch without the Teuthology testing.

Signed-off-by: Radoslaw Zarzynski <[email protected]>

Author: rzarzynski

src/rgw/rgw_zone_types.h

@@ -626,7 +626,7 @@ struct RGWZoneGroupPlacementTier {
   }
 
   void decode(bufferlist::const_iterator& bl) {
-    DECODE_START(3, bl);
+    DECODE_START(4, bl);
     decode(tier_type, bl);
     decode(storage_class, bl);
     decode(retain_head_object, bl);