expect/actual classes: experimental -> Beta

KT-61573
^KT-61712 Fixed
Review: https://jetbrains.team/p/kt/reviews/12044/files

It's a follow up commit

According our guidelines, it must be in Beta
https://kotlinlang.org/docs/components-stability.html#stability-of-subcomponents

And the whole multiplatform was in Beta, so we can't make part of the
multiplatform to have lower stability level

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/CommonCompilerArguments.kt

@@ -634,7 +634,7 @@ abstract class CommonCompilerArguments : CommonToolArguments() {
 
     @Argument(
         value = "-Xexpect-actual-classes",
-        description = "The expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature.\n" +
+        description = "'expect'/'actual' classes (including interfaces, objects, annotations, enums, and 'actual' typealiases) are in Beta.\n" +
                 "Kotlin reports a warning every time you use them. You can use this flag to mute the warning."
     )
     var expectActualClasses = false