Replace unneeded type parameter with projection

Type parameter of awaitClose is not used but produces a warning in
new type inference when used with builder inference as in callbackFlow
as seen in this issue: https://youtrack.jetbrains.com/issue/KT-32097

Using star projection removes the warning and is binary compatible
thanks to generics type erasure.

Author: LouisCAD

kotlinx-coroutines-core/common/src/channels/Produce.kt

@@ -42,7 +42,7 @@ public interface ProducerScope<in E> : CoroutineScope, SendChannel<E> {
  * ```
  */
 @ExperimentalCoroutinesApi
-public suspend fun <T> ProducerScope<T>.awaitClose(block: () -> Unit = {}) {
+public suspend fun ProducerScope<*>.awaitClose(block: () -> Unit = {}) {
     check(kotlin.coroutines.coroutineContext[Job] === this) { "awaitClose() can be invoke only from the producer context" }
     try {
         suspendCancellableCoroutine<Unit> { cont ->

kotlinx-coroutines-core/common/test/channels/ProduceTest.kt

@@ -145,7 +145,7 @@ class ProduceTest : TestBase() {
     fun testAwaitIllegalState() = runTest {
         val channel = produce<Int> {  }
         @Suppress("RemoveExplicitTypeArguments") // KT-31525
-        assertFailsWith<IllegalStateException> { (channel as ProducerScope<*>).awaitClose<Nothing>() }
+        assertFailsWith<IllegalStateException> { (channel as ProducerScope<*>).awaitClose() }
     }
 
     private suspend fun cancelOnCompletion(coroutineContext: CoroutineContext) = CoroutineScope(coroutineContext).apply {