fix: dd-smoke-tests needs to use Java 8

ProcessManager uses the `CharBuffer.flip` method, which is
coming from the super class in JDK 8 so its signature is
`()Ljava.nio.Buffer`, but since JDK9 `CharBuffer` overrides
it, in the bytecode the signatures becomes `()Ljava.nio.CharBuffer`.

This code fails when running with testJvm=8.

Author: bric3

dd-smoke-tests/build.gradle

@@ -9,6 +9,15 @@ dependencies {
   api project(':utils:test-agent-utils:decoder')
 }
 
+tasks.withType(GroovyCompile).configureEach {
+  configureCompiler(
+    it,
+    8,
+    JavaVersion.VERSION_1_8,
+    "CharBuffer#flip() signature needs to be ()Ljava.nio.Buffer (to run on JDK8)."
+    )
+}
+
 // Disable forbiddenApis for all smoke-test modules
 allprojects {
   tasks.matching { it.name.startsWith('forbiddenApis') }.configureEach {