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

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -3688,8 +3688,8 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
-    add(FirErrors.EXPECT_ACTUAL_CLASSIFIERS_ARE_EXPERIMENTAL_WARNING) { firDiagnostic ->
-        ExpectActualClassifiersAreExperimentalWarningImpl(
+    add(FirErrors.EXPECT_ACTUAL_CLASSIFIERS_ARE_IN_BETA_WARNING) { firDiagnostic ->
+        ExpectActualClassifiersAreInBetaWarningImpl(
             firDiagnostic as KtPsiDiagnostic,
             token,
         )

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -2580,8 +2580,8 @@ sealed interface KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = ActualMissing::class
     }
 
-    interface ExpectActualClassifiersAreExperimentalWarning : KtFirDiagnostic<KtClassLikeDeclaration> {
-        override val diagnosticClass get() = ExpectActualClassifiersAreExperimentalWarning::class
+    interface ExpectActualClassifiersAreInBetaWarning : KtFirDiagnostic<KtClassLikeDeclaration> {
+        override val diagnosticClass get() = ExpectActualClassifiersAreInBetaWarning::class
     }
 
     interface NotAMultiplatformCompilation : KtFirDiagnostic<PsiElement> {

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -3109,10 +3109,10 @@ internal class ActualMissingImpl(
     token: KtLifetimeToken,
 ) : KtAbstractFirDiagnostic<KtNamedDeclaration>(firDiagnostic, token), KtFirDiagnostic.ActualMissing
 
-internal class ExpectActualClassifiersAreExperimentalWarningImpl(
+internal class ExpectActualClassifiersAreInBetaWarningImpl(
     firDiagnostic: KtPsiDiagnostic,
     token: KtLifetimeToken,
-) : KtAbstractFirDiagnostic<KtClassLikeDeclaration>(firDiagnostic, token), KtFirDiagnostic.ExpectActualClassifiersAreExperimentalWarning
+) : KtAbstractFirDiagnostic<KtClassLikeDeclaration>(firDiagnostic, token), KtFirDiagnostic.ExpectActualClassifiersAreInBetaWarning
 
 internal class NotAMultiplatformCompilationImpl(
     firDiagnostic: KtPsiDiagnostic,