test(bulk-model-sync-gradle): use correct JDK version for graph-lang-api

bulk-model-sync-gradle-test specifies JDK11 so the graph-lang-api also
has to be compatible to JDK11.

Author: slisson

bulk-model-sync-gradle-test/graph-lang-api/build.gradle.kts

@@ -36,6 +36,7 @@ kotlin {
     sourceSets.named("main") {
         kotlin.srcDir(kotlinGenDir)
     }
+    jvmToolchain(11)
 }
 
 metamodel {