Force CtsNetTestCases to compile with Kotlin 1.9

The change to use Kotlin 2 by default causes NetworkAgentTest to
crash on S, possibly due to the compiler emitting synthetic methods that access API classes that don't exist on S. Force it to compile with Kotlin 1.9 until we can fix it.

Change-Id: Ief9b34c8d1eb1469ea33408736d9ca250019dbe9
Fix: 391766151
Test: Test-only change
diff --git a/tests/cts/net/Android.bp b/tests/cts/net/Android.bp
index 1ba581a..ce94f87 100644
--- a/tests/cts/net/Android.bp
+++ b/tests/cts/net/Android.bp
@@ -24,6 +24,9 @@
         "framework-connectivity-test-defaults",
     ],
 
+    // TODO(b/391766151): remove when NetworkAgentTest passes with Kotlin 2.
+    kotlin_lang_version: "1.9",
+
     // Include both the 32 and 64 bit versions
     compile_multilib: "both",