Change constructor to private for ProjectMetadata (elastic#124060)

Similar to Metadata, the constructor should be private since we expect
it to be created with the Builder.

Author: ywangd

server/src/main/java/org/elasticsearch/cluster/metadata/ProjectMetadata.java

@@ -100,7 +100,7 @@ public class ProjectMetadata implements Iterable<IndexMetadata>, Diffable<Projec
     private final IndexVersion oldestIndexVersion;
 
     @SuppressWarnings("this-escape")
-    public ProjectMetadata(
+    private ProjectMetadata(
         ProjectId id,
         ImmutableOpenMap<String, IndexMetadata> indices,
         ImmutableOpenMap<String, Set<Index>> aliasedIndices,