build(deps): bump com.google.cloud.tools:appengine-gradle-plugin

Bumps [com.google.cloud.tools:appengine-gradle-plugin](https://github.com/GoogleCloudPlatform/appengine-plugins) from 2.4.5 to 2.8.1.
- [Release notes](https://github.com/GoogleCloudPlatform/appengine-plugins/releases)
- [Changelog](https://github.com/GoogleCloudPlatform/appengine-plugins/blob/main/CHANGELOG.md)
- [Commits](https://github.com/GoogleCloudPlatform/appengine-plugins/commits/v2.8.1)

---
updated-dependencies:
- dependency-name: com.google.cloud.tools:appengine-gradle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

backend/build.gradle

@@ -17,7 +17,7 @@ buildscript {    // Configuration for building
     mavenCentral()
   }
   dependencies {
-    classpath 'com.google.cloud.tools:appengine-gradle-plugin:2.4.5'
+    classpath 'com.google.cloud.tools:appengine-gradle-plugin:2.8.1'
   }
 }