deps: update dependency com.google.cloud:google-cloud-notification to v0.121.0-beta (#772)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| com.google.cloud:google-cloud-notification | minor | `0.120.2-beta` -> `0.121.0-beta` |

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Author: renovate-bot

pom.xml

@@ -446,7 +446,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-notification</artifactId>
-        <version>0.120.2-beta</version>
+        <version>0.121.0-beta</version>
       </dependency>
       <dependency>
         <groupId>com.google.cloud</groupId>