Bump com.google.cloud.tools:appengine-gradle-plugin from 2.4.5 to 2.8.1 (#541)

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
...

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backend/build.gradle

@@ -3,7 +3,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.google.cloud.tools:appengine-gradle-plugin:2.4.5'
+        classpath 'com.google.cloud.tools:appengine-gradle-plugin:2.8.1'
     }
 }