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/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -866,7 +866,7 @@ public interface Errors {
             ACTUAL_CLASSIFIER_MUST_HAVE_THE_SAME_SUPERTYPES_AS_NON_FINAL_EXPECT_CLASSIFIER =
             DiagnosticFactory3.create(ERROR, DECLARATION_NAME);
 
-    DiagnosticFactory0<KtClassLikeDeclaration> EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING = DiagnosticFactory0.create(WARNING, EXPECT_ACTUAL_MODIFIER);
+    DiagnosticFactory0<KtClassLikeDeclaration> EXPECT_ACTUAL_CLASSIFIERS_ARE_IN_BETA_WARNING = DiagnosticFactory0.create(WARNING, EXPECT_ACTUAL_MODIFIER);
 
     DiagnosticFactory0<PsiElement> OPTIONAL_EXPECTATION_NOT_ON_EXPECTED = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<PsiElement> OPTIONAL_DECLARATION_OUTSIDE_OF_ANNOTATION_ENTRY = DiagnosticFactory0.create(ERROR);

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -430,9 +430,10 @@ public class DefaultErrorMessages {
                 new ListRenderer<>(TO_STRING, (elem) -> "'" + elem + "'"),
                 NAME);
 
-        MAP.put(EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING,
-                "The expect/actual classes (including interfaces, objects, annotations, enums, actual typealiases) are an experimental feature. " +
-                "You can use -Xexpect-actual-classes flag to suppress this warning.");
+        MAP.put(EXPECT_ACTUAL_CLASSIFIERS_ARE_IN_BETA_WARNING,
+                "'expect'/'actual' classes (including interfaces, objects, annotations, enums, and 'actual' typealiases) are in Beta. " +
+                "You can use -Xexpect-actual-classes flag to suppress this warning. " +
+                "Also see: https://youtrack.jetbrains.com/issue/KT-61573");
 
         MAP.put(OPTIONAL_EXPECTATION_NOT_ON_EXPECTED, "'@OptionalExpectation' can only be used on an expected annotation class");
         MAP.put(OPTIONAL_DECLARATION_OUTSIDE_OF_ANNOTATION_ENTRY, "Declaration annotated with '@OptionalExpectation' can only be used inside an annotation entry");

compiler/frontend/src/org/jetbrains/kotlin/resolve/PlatformConfiguratorBase.kt

@@ -17,7 +17,7 @@ import org.jetbrains.kotlin.types.DynamicTypesSettings
 private val DEFAULT_DECLARATION_CHECKERS = listOf(
     ExpectActualInTheSameModuleChecker,
     ActualClassifierMustHasTheSameMembersAsNonFinalExpectClassifierChecker,
-    ExpectActualClassifiersAreExperimentalChecker,
+    ExpectActualClassifiersAreInBetaChecker,
     DataClassDeclarationChecker(),
     ConstModifierChecker,
     UnderscoreChecker,

compiler/frontend/src/org/jetbrains/kotlin/resolve/checkers/ExpectActualClassifiersAreInBetaChecker.kt

@@ -17,7 +17,7 @@ import org.jetbrains.kotlin.psi.KtClassOrObject
 import org.jetbrains.kotlin.psi.KtDeclaration
 import org.jetbrains.kotlin.psi.KtTypeAlias
 
-object ExpectActualClassifiersAreExperimentalChecker : DeclarationChecker {
+object ExpectActualClassifiersAreInBetaChecker : DeclarationChecker {
     override fun check(declaration: KtDeclaration, descriptor: DeclarationDescriptor, context: DeclarationCheckerContext) {
         if (!context.languageVersionSettings.supportsFeature(LanguageFeature.MultiPlatformProjects)) return
         if (context.languageVersionSettings.getFlag(AnalysisFlags.muteExpectActualClassesWarning)) return
@@ -31,7 +31,7 @@ object ExpectActualClassifiersAreExperimentalChecker : DeclarationChecker {
         check(descriptor is ClassifierDescriptorWithTypeParameters)
 
         if (descriptor.isExpect || descriptor.isActual) {
-            context.trace.report(Errors.EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING.on(declaration))
+            context.trace.report(Errors.EXPECT_ACTUAL_CLASSIFIERS_ARE_IN_BETA_WARNING.on(declaration))
         }
     }
 }