Remove redundant list copy in CanMatchNodeRequest (elastic#121700)

We're not mutating that list ever, so lets just use an immutable list
all the way here and avoid at least one round of needless copy.

Author: original-brownbear

server/src/main/java/org/elasticsearch/action/search/CanMatchNodeRequest.java

@@ -136,7 +136,7 @@ public CanMatchNodeRequest(
     ) {
         this.source = getCanMatchSource(searchRequest);
         this.indicesOptions = indicesOptions;
-        this.shards = new ArrayList<>(shards);
+        this.shards = shards;
         this.searchType = searchRequest.searchType();
         this.requestCache = searchRequest.requestCache();
         // If allowPartialSearchResults is unset (ie null), the cluster-level default should have been substituted