Deprecate HashSet.getElement() and document that it's for ObjC interop.

Will become HIDDEN eventually.

As a part of efforts to stabilize Native #KT-55765.

libraries/stdlib/native-wasm/src/kotlin/collections/HashSet.kt

@@ -63,6 +63,10 @@ actual class HashSet<E> internal constructor(
     override actual val size: Int get() = backing.size
     override actual fun isEmpty(): Boolean = backing.isEmpty()
     override actual fun contains(element: E): Boolean = backing.containsKey(element)
+
+    /** Implements KonanSet.getElement(). Used for ObjC interop. */
+    @Deprecated("This function is not supposed to be used directly.")
+    @DeprecatedSinceKotlin(warningSince = "1.9") // TODO: advance to HIDDEN eventually
     override fun getElement(element: E): E? = backing.getKey(element)
     override actual fun clear() = backing.clear()
     override actual fun add(element: E): Boolean = backing.addKey(element) >= 0