fix(deps): update dependency com.google.appengine:appengine-api-1.0-sdk to v2.0.21 (#2371)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.appengine:appengine-api-1.0-sdk](https://togithub.com/GoogleCloudPlatform/appengine-java-standard) | `2.0.19` -> `2.0.21` | [![age](https://developer.mend.io/api/mc/badges/age/maven/com.google.appengine:appengine-api-1.0-sdk/2.0.21?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://developer.mend.io/api/mc/badges/adoption/maven/com.google.appengine:appengine-api-1.0-sdk/2.0.21?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://developer.mend.io/api/mc/badges/compatibility/maven/com.google.appengine:appengine-api-1.0-sdk/2.0.19/2.0.21?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://developer.mend.io/api/mc/badges/confidence/maven/com.google.appengine:appengine-api-1.0-sdk/2.0.19/2.0.21?slim=true)](https://docs.renovatebot.com/merge-confidence/) |

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> Some dependencies could not be looked up. Check the Dependency Dashboard for more information.

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### Release Notes

<details>
<summary>GoogleCloudPlatform/appengine-java-standard (com.google.appengine:appengine-api-1.0-sdk)</summary>

### [`v2.0.21`](https://togithub.com/GoogleCloudPlatform/appengine-java-standard/releases/tag/v2.0.21)

[Compare Source](https://togithub.com/GoogleCloudPlatform/appengine-java-standard/compare/v2.0.20...v2.0.21)

**Full Changelog**: https://togithub.com/GoogleCloudPlatform/appengine-java-standard/compare/v2.0.20...v2.0.21

### [`v2.0.20`](https://togithub.com/GoogleCloudPlatform/appengine-java-standard/releases/tag/v2.0.20)

[Compare Source](https://togithub.com/GoogleCloudPlatform/appengine-java-standard/compare/v2.0.19...v2.0.20)

**Full Changelog**: https://togithub.com/GoogleCloudPlatform/appengine-java-standard/compare/v2.0.18...v2.0.20

</details>

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Author: renovate-bot

pom.xml

@@ -503,7 +503,7 @@
     <project.gson.version>2.8.6</project.gson.version>
     <project.protobuf-java.version>3.24.4</project.protobuf-java.version>
     <project.guava.version>32.0.0-jre</project.guava.version>
-    <project.appengine.version>2.0.19</project.appengine.version>
+    <project.appengine.version>2.0.21</project.appengine.version>
     <project.xpp3.version>1.1.4c</project.xpp3.version>
     <project.jdo2-api.version>2.3-20090302111651</project.jdo2-api.version>
     <project.datanucleus-core.version>3.2.2</project.datanucleus-core.version>