deps: update dependency com.google.code.gson:gson to v2.8.9 (#357)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.code.gson:gson](https://togithub.com/google/gson) | `2.8.7` -> `2.8.9` | [![age](https://badges.renovateapi.com/packages/maven/com.google.code.gson:gson/2.8.9/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.code.gson:gson/2.8.9/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.code.gson:gson/2.8.9/compatibility-slim/2.8.7)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.code.gson:gson/2.8.9/confidence-slim/2.8.7)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v2.8.9`](https://togithub.com/google/gson/blob/master/CHANGELOG.md#Version-289)

-   Make OSGi bundle's dependency on `sun.misc` optional ([#&#8203;1993](https://togithub.com/google/gson/issues/1993)).
-   Deprecate `Gson.excluder()` exposing internal `Excluder` class ([#&#8203;1986](https://togithub.com/google/gson/issues/1986)).
-   Prevent Java deserialization of internal classes ([#&#8203;1991](https://togithub.com/google/gson/issues/1991)).
-   Improve number strategy implementation ([#&#8203;1987](https://togithub.com/google/gson/issues/1987)).
-   Fix LongSerializationPolicy null handling being inconsistent with Gson ([#&#8203;1990](https://togithub.com/google/gson/issues/1990)).
-   Support arbitrary Number implementation for Object and Number deserialization ([#&#8203;1290](https://togithub.com/google/gson/issues/1290)).
-   Bump proguard-maven-plugin from 2.4.0 to 2.5.1 ([#&#8203;1980](https://togithub.com/google/gson/issues/1980)).
-   Don't exclude static local classes ([#&#8203;1969](https://togithub.com/google/gson/issues/1969)).
-   Fix `RuntimeTypeAdapterFactory` depending on internal `Streams` class ([#&#8203;1959](https://togithub.com/google/gson/issues/1959)).
-   Improve Maven build ([#&#8203;1964](https://togithub.com/google/gson/issues/1964)).
-   Make dependency on `java.sql` optional ([#&#8203;1707](https://togithub.com/google/gson/issues/1707)).

### [`v2.8.8`](https://togithub.com/google/gson/blob/master/CHANGELOG.md#Version-288)

-   Fixed issue with recursive types ([#&#8203;1390](https://togithub.com/google/gson/issues/1390)).
-   Better behaviour with Java 9+ and `Unsafe` if there is a security manager ([#&#8203;1712](https://togithub.com/google/gson/issues/1712)).
-   `EnumTypeAdapter` now works better when ProGuard has obfuscated enum fields ([#&#8203;1495](https://togithub.com/google/gson/issues/1495)).

</details>

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Author: renovate-bot

dialogflow-cx/snippets/pom.xml

@@ -33,7 +33,7 @@
     <dependency>
       <groupId>com.google.code.gson</groupId>
       <artifactId>gson</artifactId>
-      <version>2.8.7</version>
+      <version>2.8.9</version>
     </dependency>
     <dependency>
       <groupId>com.google.cloud.functions</groupId>