Add scripted test for #6175 (#8598)

This PR adds a scripted test to prevent regression of issue #6175, where `MethodHandle.invokeExact` failed to compile on JDK 15+ when using `-release 11`.

Changes:
- Added `sbt-app/src/sbt-test/java/method-handle-release/` scripted test
- Test uses Scala 2.13.16 with `-release:11` to reproduce the original issue scenario
- Verifies that polymorphic signature methods compile correctly under cross-version targeting

sbt-app/src/sbt-test/java/method-handle-release/build.sbt

@@ -0,0 +1,2 @@
+ThisBuild / scalaVersion := "2.13.16"
+scalacOptions += "-release:11"

sbt-app/src/sbt-test/java/method-handle-release/src/main/scala/Main.scala

@@ -0,0 +1,19 @@
+import java.lang.invoke.{ MethodHandle, MethodHandles }
+
+object Main {
+  case class TestClass(number: Int)
+  def testMethod(): TestClass = {
+    TestClass(10)
+  }
+
+  def wrappedMethod(handle: MethodHandle): TestClass = {
+    handle.invokeExact()
+  }
+
+  def main(args: Array[String]): Unit = {
+    val method = classOf[Main.type].getDeclaredMethod("testMethod")
+    method.setAccessible(true)
+    val handle = MethodHandles.lookup.unreflect(method)
+    println(wrappedMethod(handle))
+  }
+}

sbt-app/src/sbt-test/java/method-handle-release/test

@@ -0,0 +1,3 @@
+# https://github.com/sbt/sbt/issues/6175
+# MethodHandle.invokeExact fails to compile on JDK 15+ when targeting Java 11
+> compile