Bump graphql-spring-boot-starter in /plugins/maven/example-server

Bumps [graphql-spring-boot-starter](https://github.com/graphql-java-kickstart/graphql-spring-boot) from 11.0.0 to 15.0.0.
- [Release notes](https://github.com/graphql-java-kickstart/graphql-spring-boot/releases)
- [Commits](graphql-java-kickstart/graphql-spring-boot@v11.0.0...v15.0.0)

---
updated-dependencies:
- dependency-name: com.graphql-java-kickstart:graphql-spring-boot-starter
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

plugins/maven/example-server/pom.xml

@@ -87,7 +87,7 @@
         <dependency>
             <groupId>com.graphql-java-kickstart</groupId>
             <artifactId>graphql-spring-boot-starter</artifactId>
-            <version>11.0.0</version>
+            <version>15.0.0</version>
         </dependency>
         <dependency>
             <groupId>com.graphql-java-kickstart</groupId>