deps: update project.appengine.version to v1.9.88

Author: renovate-bot

pom.xml

@@ -578,7 +578,7 @@
       - Internally, update the default features.json file
     -->
     <project.http-client.version>1.39.2</project.http-client.version><!-- {x-version-update:google-http-client-parent:current} -->
-    <project.appengine.version>1.9.71</project.appengine.version>
+    <project.appengine.version>1.9.88</project.appengine.version>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <project.jsr305.version>3.0.2</project.jsr305.version>
     <project.gson.version>2.8.6</project.gson.version>