RateLimiterProcessing: Adapt to older gson version

Gerrit 3.5 is using older gson version 2.8.5, without this commit: [1].
Adapt the code correspondingly. Note, that this change should be
reverted when merged up to master branch.

[1] https://github.com/google/gson/commit/c5a3f21fbad3828deba1deb88168e1f76829f807

Depends-On: https://gerrit-review.googlesource.com/c/zuul/jobs/+/339536
Change-Id: I5ae5a88fc4f48296424001732938b4ccf42e9975
diff --git a/src/main/java/com/googlesource/gerrit/plugins/ratelimiter/RateLimiterProcessing.java b/src/main/java/com/googlesource/gerrit/plugins/ratelimiter/RateLimiterProcessing.java
index 8f23613..8e11de9 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/ratelimiter/RateLimiterProcessing.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/ratelimiter/RateLimiterProcessing.java
@@ -67,7 +67,7 @@
         .sorted(Map.Entry.comparingByValue())
         .map(this::getJsonObjectString)
         .forEach(permitList::add);
-    return gson.toJson(JsonParser.parseString(permitList.toString()));
+    return gson.toJson(new JsonParser().parse(permitList.toString()));
   }
 
   private String getJsonObjectString(Map.Entry<String, RateLimiter> entry) {