Deprecate more fields in RegistryConfig

This primarily addresses issues with TMCH testing mode and email sending utils.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=143710550

java/google/registry/config/RegistryConfigLoader.java

@@ -28,6 +28,7 @@ import javax.annotation.concurrent.ThreadSafe;
  * passing the {@link RegistryEnvironment} as a single parameter.
  */
 @ThreadSafe
+@Deprecated // will be replaced by YAML config; see b/33386530 for details
 public final class RegistryConfigLoader {
 
   public static final String REGISTRY_CONFIG_PROPERTY = "google.registry.config";