deps: update dependency com.google.code.gson:gson to v2.9.0 (#422)

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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) | `2.8.9` -> `2.9.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.code.gson:gson/2.9.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.code.gson:gson/2.9.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.code.gson:gson/2.9.0/compatibility-slim/2.8.9)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.code.gson:gson/2.9.0/confidence-slim/2.8.9)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v2.9.0`](https://togithub.com/google/gson/blob/HEAD/CHANGELOG.md#Version-290)

**The minimum supported Java version changes from 6 to 7.**

-   Change target Java version to 7 ([#&#8203;2043](https://togithub.com/google/gson/issues/2043))
-   Put `module-info.class` into Multi-Release JAR folder ([#&#8203;2013](https://togithub.com/google/gson/issues/2013))
-   Improve error message when abstract class cannot be constructed ([#&#8203;1814](https://togithub.com/google/gson/issues/1814))
-   Support EnumMap deserialization ([#&#8203;2071](https://togithub.com/google/gson/issues/2071))
-   Add LazilyParsedNumber default adapter ([#&#8203;2060](https://togithub.com/google/gson/issues/2060))
-   Fix JsonReader.hasNext() returning true at end of document ([#&#8203;2061](https://togithub.com/google/gson/issues/2061))
-   Remove Gradle build support. Build script was outdated and not actively
    maintained anymore ([#&#8203;2063](https://togithub.com/google/gson/issues/2063))
-   Add `GsonBuilder.disableJdkUnsafe()` ([#&#8203;1904](https://togithub.com/google/gson/issues/1904))
-   Add `UPPER_CASE_WITH_UNDERSCORES` in FieldNamingPolicy ([#&#8203;2024](https://togithub.com/google/gson/issues/2024))
-   Fix failing to serialize Collection or Map with inaccessible constructor ([#&#8203;1902](https://togithub.com/google/gson/issues/1902))
-   Improve TreeTypeAdapter thread-safety ([#&#8203;1976](https://togithub.com/google/gson/issues/1976))
-   Fix `Gson.newJsonWriter` ignoring lenient and HTML-safe setting ([#&#8203;1989](https://togithub.com/google/gson/issues/1989))
-   Delete unused LinkedHashTreeMap ([#&#8203;1992](https://togithub.com/google/gson/issues/1992))
-   Make default adapters stricter; improve exception messages ([#&#8203;2000](https://togithub.com/google/gson/issues/2000))
-   Fix `FieldNamingPolicy.upperCaseFirstLetter` uppercasing non-letter ([#&#8203;2004](https://togithub.com/google/gson/issues/2004))

</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>2.8.9</version>
+      <version>2.9.0</version>
     </dependency>
     <dependency>
       <groupId>com.google.cloud.functions</groupId>