[K/N] Deprecated freezing ^KT-50541

Starting with 1.7.20 freezing is deprecated. See https://github.com/JetBrains/kotlin/blob/master/kotlin-native/NEW_MM.md#freezing-deprecation for details.

Merge-request: KT-MR-6399
Merged-by: Alexander Shabalin <Alexander.Shabalin@jetbrains.com>

kotlin-native/backend.native/tests/stdlib_external/text/StringEncodingTestNative.kt

@@ -2,6 +2,7 @@
  * Copyright 2010-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
  * that can be found in the LICENSE file.
  */
+@file:OptIn(FreezingIsDeprecated::class)
 
 package test.text