Polish ClientRegistrations

Class is now final

Issue: gh-5647

oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/registration/ClientRegistrations.java

@@ -39,7 +39,7 @@ import org.springframework.web.client.RestTemplate;
  * @author Josh Cummings
  * @since 5.1
  */
-public class ClientRegistrations {
+public final class ClientRegistrations {
 
 	/**
 	 * Creates a {@link ClientRegistration.Builder}  using the provided