Mark enum field on CreditType as final

Enums should be immutable, so all fields on them should be final.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=142025872

java/google/registry/model/billing/RegistrarCredit.java

@@ -55,7 +55,7 @@ public final class RegistrarCredit extends ImmutableObject implements Buildable
     PROMOTION("Promotional Credit");
 
     /** A descriptive name for a credit of this type. */
-    private String descriptiveName;
+    private final String descriptiveName;
 
     CreditType(String descriptiveName) {
       this.descriptiveName = descriptiveName;