deps: update project.appengine.version to v2.0.9 (#1753)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.appengine:appengine-api-stubs](https://togithub.com/GoogleCloudPlatform/appengine-java-standard) | `2.0.8` -> `2.0.9` | [![age](https://badges.renovateapi.com/packages/maven/com.google.appengine:appengine-api-stubs/2.0.9/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.appengine:appengine-api-stubs/2.0.9/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.appengine:appengine-api-stubs/2.0.9/compatibility-slim/2.0.8)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.appengine:appengine-api-stubs/2.0.9/confidence-slim/2.0.8)](https://docs.renovatebot.com/merge-confidence/) |
| [com.google.appengine:appengine-testing](https://togithub.com/GoogleCloudPlatform/appengine-java-standard) | `2.0.8` -> `2.0.9` | [![age](https://badges.renovateapi.com/packages/maven/com.google.appengine:appengine-testing/2.0.9/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.appengine:appengine-testing/2.0.9/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.appengine:appengine-testing/2.0.9/compatibility-slim/2.0.8)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.appengine:appengine-testing/2.0.9/confidence-slim/2.0.8)](https://docs.renovatebot.com/merge-confidence/) |
| [com.google.appengine:appengine-api-1.0-sdk](https://togithub.com/GoogleCloudPlatform/appengine-java-standard) | `2.0.8` -> `2.0.9` | [![age](https://badges.renovateapi.com/packages/maven/com.google.appengine:appengine-api-1.0-sdk/2.0.9/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.appengine:appengine-api-1.0-sdk/2.0.9/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.appengine:appengine-api-1.0-sdk/2.0.9/compatibility-slim/2.0.8)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.appengine:appengine-api-1.0-sdk/2.0.9/confidence-slim/2.0.8)](https://docs.renovatebot.com/merge-confidence/) |

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pom.xml

@@ -561,7 +561,7 @@
       - Internally, update the default features.json file
     -->
     <project.http-client.version>1.42.3-SNAPSHOT</project.http-client.version><!-- {x-version-update:google-http-client-parent:current} -->
-    <project.appengine.version>2.0.8</project.appengine.version>
+    <project.appengine.version>2.0.9</project.appengine.version>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <project.jsr305.version>3.0.2</project.jsr305.version>
     <project.gson.version>2.9.1</project.gson.version>