Fix SingletonIdentityCacheKey#equals

It made caching with `SingletonIdentityCacheMapping` ineffective,
especially if the key is embedded within a compund key, as the singleton
key was never reported equal to any other key. There is only one user of
this class, and it doesn't rely on key equality, so the impact is
minimal.

plugin/trino-base-jdbc/src/main/java/io/trino/plugin/jdbc/SingletonIdentityCacheMapping.java

@@ -38,7 +38,7 @@ public int hashCode()
         @Override
         public boolean equals(Object obj)
         {
-            return obj instanceof SingletonIdentityCacheMapping;
+            return obj instanceof SingletonIdentityCacheKey;
         }
     }
 }