fix(gradle-model): Ensure compatibility by lowering the Java target

Signed-off-by: Sebastian Schuberth <sebastian@doubleopen.org>

plugins/package-managers/gradle-model/build.gradle.kts

@@ -17,8 +17,26 @@
  * License-Filename: LICENSE
  */
 
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
 plugins {
     // Apply precompiled plugins.
     id("ort-kotlin-conventions")
     id("ort-publication-conventions")
 }
+
+// Classes that are sent to the build via custom build actions need to target the lowest supported Java version, which
+// is Java 8 for Gradle 5 and above, see
+// https://docs.gradle.org/current/userguide/third_party_integration.html#sec:embedding_compatibility
+val gradleToolingApiLowestSupportedJavaVersion = JvmTarget.JVM_1_8
+
+tasks.named<JavaCompile>("compileJava") {
+    targetCompatibility = gradleToolingApiLowestSupportedJavaVersion.target
+}
+
+tasks.named<KotlinCompile>("compileKotlin") {
+    compilerOptions {
+        jvmTarget = gradleToolingApiLowestSupportedJavaVersion
+    }
+}