fix test app for java8 compat

Author: SylvainJuge

jmx-scraper/test-app/build.gradle.kts

@@ -4,6 +4,12 @@ plugins {
 
 description = "JMX metrics scraper - test application"
 
+java {
+  toolchain {
+    languageVersion = JavaLanguageVersion.of(8)
+  }
+}
+
 tasks {
   jar {
     manifest {

jmx-scraper/test-webapp/build.gradle.kts

@@ -10,6 +10,6 @@ dependencies {
 
 java {
   toolchain {
-    languageVersion = JavaLanguageVersion.of(11)
+    languageVersion = JavaLanguageVersion.of(8)
   }
 }