Use Java 11 in :jps:jps-plugin module in order to fix compilation

IntelliJ uses Java 11 so it forces us to use Java 11 in JPS

This commit fixes:
```
public class DataContainerVersionChangedTestGenerated extends AbstractDataContainerVersionChangedTest {
       ^
  bad class file: /home/bobko/.gradle/caches/modules-2/files-2.1/com.jetbrains.intellij.platform/test-framework/203.8084.24/ba5c6b3b0eae0a083dc30e866c362094f5072853/test-framework-203.8084.24.jar(com/intellij/testFramework/UsefulTestCase.class)
    class file has wrong version 55.0, should be 52.0
    Please remove or make sure it appears in the correct subdirectory of the classpath.
```

jps/jps-plugin/build.gradle.kts

@@ -61,6 +61,12 @@ sourceSets {
     }
 }
 
+java {
+    toolchain {
+        languageVersion.set(JavaLanguageVersion.of(11))
+    }
+}
+
 projectTest(parallel = true) {
     // do not replace with compile/runtime dependency,
     // because it forces Intellij reindexing after each compiler change