Bump graphql-spring-boot-starter in /plugins/gradle/example-server (#349)

Bumps [graphql-spring-boot-starter](https://github.com/graphql-java-kickstart/graphql-spring-boot) from 7.1.0 to 8.0.0.
- [Release notes](https://github.com/graphql-java-kickstart/graphql-spring-boot/releases)
- [Commits](https://github.com/graphql-java-kickstart/graphql-spring-boot/compare/v7.1.0...v8.0.0)
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Co-authored-by: Ndependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

plugins/gradle/example-server/build.gradle

@@ -15,7 +15,7 @@ dependencies {
     implementation "org.springframework.boot:spring-boot-starter-web:2.3.4.RELEASE"
     implementation "org.springframework.boot:spring-boot-starter-data-mongodb:2.3.4.RELEASE"
 
-    implementation "com.graphql-java-kickstart:graphql-spring-boot-starter:7.1.0"
+    implementation "com.graphql-java-kickstart:graphql-spring-boot-starter:8.0.0"
     implementation "com.graphql-java-kickstart:graphiql-spring-boot-starter:8.0.0"
     implementation "com.graphql-java:graphql-java-extended-scalars:1.1.0"