Bump graphql-java from 14.0 to 15.0 (#183)

Author: dependabot[bot]

build.gradle

@@ -18,7 +18,7 @@ repositories {
 
 dependencies {
     implementation "org.freemarker:freemarker:2.3.30"
-    implementation "com.graphql-java:graphql-java:14.0"
+    implementation "com.graphql-java:graphql-java:15.0"
     implementation "com.fasterxml.jackson.core:jackson-databind:2.10.3"
 
     implementation "org.projectlombok:lombok:1.18.12"