chore(deps): update dependency com.google.cloud:google-cloud-notification to v0.121.5-beta (#217)

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This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.cloud:google-cloud-notification](com/google/cloud/google-cloud-notification) | patch | `0.121.3-beta` -> `0.121.5-beta` |

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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>0.121.3-beta</version>
+      <version>0.121.5-beta</version>
     </dependency>
 
     <dependency>