deps: update dependency com.google.code.gson:gson to v2.9.1 (#528)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.code.gson:gson](https://github.com/google/gson) | `2.9.0` -> `2.9.1` | [![age](https://badges.renovateapi.com/packages/maven/com.google.code.gson:gson/2.9.1/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.code.gson:gson/2.9.1/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.code.gson:gson/2.9.1/compatibility-slim/2.9.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.code.gson:gson/2.9.1/confidence-slim/2.9.0)](https://docs.renovatebot.com/merge-confidence/) |

---

### Release Notes

<details>
<summary>google/gson</summary>

### [`v2.9.1`](https://github.com/google/gson/blob/HEAD/CHANGELOG.md#Version-291)

-   Make `Object` and `JsonElement` deserialization iterative rather than
    recursive ([#&#8203;1912](https://github.com/google/gson/issues/1912))
-   Added parsing support for enum that has overridden toString() method ([#&#8203;1950](https://github.com/google/gson/issues/1950))
-   Removed support for building Gson with Gradle ([#&#8203;2081](https://github.com/google/gson/issues/2081))
-   Removed obsolete `codegen` hierarchy ([#&#8203;2099](https://github.com/google/gson/issues/2099))
-   Add support for reflection access filter ([#&#8203;1905](https://github.com/google/gson/issues/1905))
-   Improve `TypeToken` creation validation ([#&#8203;2072](https://github.com/google/gson/issues/2072))
-   Add explicit support for `float` in `JsonWriter` ([#&#8203;2130](https://github.com/google/gson/issues/2130), [#&#8203;2132](https://github.com/google/gson/issues/2132))
-   Fail when parsing invalid local date ([#&#8203;2134](https://github.com/google/gson/issues/2134))

Also many small improvements to javadoc.

</details>

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dialogflow-cx/snippets/pom.xml

@@ -33,7 +33,7 @@
     <dependency>
       <groupId>com.google.code.gson</groupId>
       <artifactId>gson</artifactId>
-      <version>2.9.0</version>
+      <version>2.9.1</version>
     </dependency>
     <dependency>
       <groupId>com.google.cloud.functions</groupId>