chore(deps): update dependency com.google.auth:google-auth-library-appengine to v0.21.0 (#3242)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| com.google.auth:google-auth-library-appengine | minor | `0.20.0` -> `0.21.0` |

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Author: renovate-bot

auth/pom.xml

@@ -60,7 +60,7 @@ limitations under the License.
     <dependency>
       <groupId>com.google.auth</groupId>
       <artifactId>google-auth-library-appengine</artifactId>
-      <version>0.20.0</version>
+      <version>0.21.0</version>
     </dependency>
     <!-- END dependencies -->
     <dependency>