KT-52217 Rename 'use-fir' to 'use-k2', update message

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

@@ -294,10 +294,11 @@ abstract class CommonCompilerArguments : CommonToolArguments() {
 
     @GradleOption(DefaultValues.BooleanFalseDefault::class)
     @Argument(
-        value = "-Xuse-fir",
-        description = "Compile using Front-end IR. Warning: this feature is far from being production-ready"
+        value = "-Xuse-k2",
+        deprecatedName = "-Xuse-fir",
+        description = "Compile using experimental K2. K2 is a new compiler pipeline, no compatibility guarantees are yet provided"
     )
-    var useFir: Boolean by FreezableVar(false)
+    var useK2: Boolean by FreezableVar(false)
 
     @Argument(
         value = "-Xuse-fir-extended-checkers",

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JVMCompilerArguments.kt

@@ -528,7 +528,7 @@ Also sets `-jvm-target` value equal to the selected JDK version"""
         result[JvmAnalysisFlags.sanitizeParentheses] = sanitizeParentheses
         result[JvmAnalysisFlags.suppressMissingBuiltinsError] = suppressMissingBuiltinsError
         result[JvmAnalysisFlags.enableJvmPreview] = enableJvmPreview
-        result[AnalysisFlags.allowUnstableDependencies] = allowUnstableDependencies || useFir
+        result[AnalysisFlags.allowUnstableDependencies] = allowUnstableDependencies || useK2
         result[JvmAnalysisFlags.disableUltraLightClasses] = disableUltraLightClasses
         result[JvmAnalysisFlags.useIR] = !useOldBackend
         return result