scpb: ensure IndexZoneConfig fields do not overlap

In cockroachdb#134522, we added a fix to ensure that we cleaned up backrefs
properly for our `IndexZoneConfig` element, but in doing so, we
created an overlap of field ID `3` between v24.2+.
This patch ensures that we do not re-use the same field ID for
a different field in our IndexZoneConfig element by marking it
as `reserved`.

Fixes: cockroachdb#133003
Fixes: cockroachdb#135807

Release note: None

pkg/sql/schemachanger/scpb/elements.proto

@@ -785,13 +785,14 @@ message TableZoneConfig {
 message IndexZoneConfig {
   uint32 table_id = 1 [(gogoproto.customname) = "TableID", (gogoproto.casttype) = "github.com/cockroachdb/cockroach/pkg/sql/sem/catid.DescID"];
   uint32 index_id = 2 [(gogoproto.customname) = "IndexID", (gogoproto.casttype) = "github.com/cockroachdb/cockroach/pkg/sql/sem/catid.IndexID"];
-  cockroach.config.zonepb.Subzone subzone = 3 [(gogoproto.nullable) = false];
-  repeated cockroach.config.zonepb.SubzoneSpan subzone_spans = 4 [(gogoproto.nullable) = false];
+  cockroach.config.zonepb.Subzone subzone = 4 [(gogoproto.nullable) = false];
+  repeated cockroach.config.zonepb.SubzoneSpan subzone_spans = 5 [(gogoproto.nullable) = false];
   // `seq_num` is used to differentiate different subzone config elements tied
   // to the same index. E.g. If we attempt to update an index subzone config,
   // our solution is to drop the existing element and add a new element, with
   // different `seq_num`.
-  uint32 seq_num = 5;
+  uint32 seq_num = 6;
+  reserved 3;
 }
 
 message PartitionZoneConfig {