chore(deps): update dependency com.google.cloud:google-cloud-notification to v0.122.21-beta (#538)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.cloud:google-cloud-notification](https://togithub.com/googleapis/java-notification) | `0.122.20-beta` -> `0.122.21-beta` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-notification/0.122.21-beta/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-notification/0.122.21-beta/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-notification/0.122.21-beta/compatibility-slim/0.122.20-beta)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-notification/0.122.21-beta/confidence-slim/0.122.20-beta)](https://docs.renovatebot.com/merge-confidence/) |

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notification/snippets/pom.xml

@@ -40,7 +40,7 @@
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-notification</artifactId>
       <!-- TODO: REMOVE VERSION SPECIFIED BELOW WHEN THE CLIENT GOES GA-->
-      <version>0.122.20-beta</version>
+      <version>0.122.21-beta</version>
     </dependency>
 
     <dependency>