fix: update Publish retry backoff settings (googleapis#1686)

* Update Publisher.java

Change default RetrySettings to use a multiplier of 4 instead of 1.3, to address aggressive retry on RESOURCE_EXHAUSTED errors. (Unfortunately, we can't set different retrySettings per error code.)

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google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/Publisher.java

@@ -686,7 +686,7 @@ public static final class Builder {
     private static final Duration DEFAULT_TOTAL_TIMEOUT = Duration.ofSeconds(600);
     private static final Duration DEFAULT_INITIAL_RETRY_DELAY = Duration.ofMillis(100);
     private static final Duration DEFAULT_MAX_RETRY_DELAY = Duration.ofSeconds(60);
-    private static final double DEFAULT_MULTIPLIER = 1.3;
+    private static final double DEFAULT_MULTIPLIER = 4;
     static final BatchingSettings DEFAULT_BATCHING_SETTINGS =
         BatchingSettings.newBuilder()
             .setDelayThreshold(DEFAULT_DELAY_THRESHOLD)