Compile Scala using Java 8 toolchain

Author: emlun

buildSrc/src/main/kotlin/project-convention-scala.gradle.kts

@@ -7,5 +7,8 @@ tasks.withType(ScalaCompile::class) {
     // See: https://github.com/gradle/gradle/issues/23193
     // See: https://github.com/gradle/gradle/pull/23198
     // See: https://github.com/gradle/gradle/pull/23751
-    scalaCompileOptions.additionalParameters = mutableListOf("-Wunused", "-release", "8", "-target:8")
+    scalaCompileOptions.additionalParameters = mutableListOf("-Wunused")
+    javaLauncher.set(javaToolchains.launcherFor {
+        languageVersion.set(JavaLanguageVersion.of(8))
+    })
 }