Bump the number of retries in transaction failures

This can potentially help even more with serializable transaction
failures (optimistic locking exceptions, which are expected to occur
somewhat frequently).

With six attempts, we will sleep at most five times, for
100+200+400+800+1600 ms each, for a total of at most 3.1 seconds (much
less than the EPP maximum which I believe (?) to be 30 seconds.

core/src/main/java/google/registry/persistence/transaction/JpaTransactionManagerImpl.java

@@ -86,7 +86,7 @@
 public class JpaTransactionManagerImpl implements JpaTransactionManager {
 
   private static final FluentLogger logger = FluentLogger.forEnclosingClass();
-  private static final Retrier retrier = new Retrier(new SystemSleeper(), 3);
+  private static final Retrier retrier = new Retrier(new SystemSleeper(), 6);
   private static final String NESTED_TRANSACTION_MESSAGE =
       "Nested transaction detected. Try refactoring to avoid nested transactions. If unachievable,"
           + " use reTransact() in nested transactions";