chore(deps): update dependency com.google.appengine:appengine-api-1.0-sdk to v2.0.21 (#1293)

Co-authored-by: Lawrence Qiu <lawrenceqiu@google.com>

Author: renovate-bot

google-auth-library-java/pom.xml

@@ -66,7 +66,7 @@
     <project.google.http.version>1.43.3</project.google.http.version>
     <project.junit.version>4.13.2</project.junit.version>
     <project.guava.version>32.0.0-android</project.guava.version>
-    <project.appengine.version>2.0.19</project.appengine.version>
+    <project.appengine.version>2.0.21</project.appengine.version>
     <project.findbugs.version>3.0.2</project.findbugs.version>
     <deploy.autorelease>false</deploy.autorelease>
     <project.autovalue.version>1.8.2</project.autovalue.version>