Bump com.google.code.gson:gson in /sparkjava-mysql/backend

Bumps [com.google.code.gson:gson](https://github.com/google/gson) from 2.8.4 to 2.8.9.
- [Release notes](https://github.com/google/gson/releases)
- [Changelog](https://github.com/google/gson/blob/main/CHANGELOG.md)
- [Commits](https://github.com/google/gson/compare/gson-parent-2.8.4...gson-parent-2.8.9)

---
updated-dependencies:
- dependency-name: com.google.code.gson:gson
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

sparkjava-mysql/backend/pom.xml

@@ -51,7 +51,7 @@
         <dependency>
             <groupId>com.google.code.gson</groupId>
             <artifactId>gson</artifactId>
-            <version>2.8.4</version>
+            <version>2.8.9</version>
         </dependency>
 
     </dependencies>