Add missing checked exception to GCSA override

This commit adds a missing checked exception declared on
TransportReplicationAction#shardOperationOnPrimary but not declared on
the overridden method
GlobalCheckpointSyncAction#shardOperationOnPrimary.

core/src/main/java/org/elasticsearch/index/seqno/GlobalCheckpointSyncAction.java

@@ -66,7 +66,7 @@ protected ReplicationResponse newResponseInstance() {
     }
 
     @Override
-    protected PrimaryResult shardOperationOnPrimary(PrimaryRequest request) {
+    protected PrimaryResult shardOperationOnPrimary(PrimaryRequest request) throws Exception {
         IndexService indexService = indicesService.indexServiceSafe(request.shardId().getIndex());
         IndexShard indexShard = indexService.getShard(request.shardId().id());
         long checkpoint = indexShard.getGlobalCheckpoint();