Clean up some code quality issues

This removes some qualifiers that aren't necessary (e.g. public/abstract on interfaces, private on enum constructors, final on private methods, static on nested interfaces/enums), uses Java 8 lambdas and features where that's an improvement

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177182945

java/google/registry/model/annotations/NotBackedUp.java

@@ -30,7 +30,7 @@ public @interface NotBackedUp {
   Reason reason();
 
   /** Reasons why a given entity does not need to be be backed up. */
-  public enum Reason {
+  enum Reason {
     /** This entity is transient by design and has only a short-term useful lifetime. */
     TRANSIENT,