update: update js-ir-compiler.md wording

docs/topics/js/js-ir-compiler.md

@@ -26,7 +26,9 @@ kotlin {
 * `LEGACY` uses the old compiler backend.
 * `BOTH` compiles your project with the new IR compiler as well as the default compiler backend. Use this mode for [authoring libraries compatible with both backends](#authoring-libraries-for-the-ir-compiler-with-backwards-compatibility).
 
-The old compiler backend is deprecated since Kotlin 1.8.0. Starting with Kotlin 1.9.0 using compiler types `LEGACY` of `BOTH` leads to an error.
+> The old compiler backend has been deprecated since Kotlin 1.8.0. Starting with Kotlin 1.9.0, using compiler types `LEGACY` of `BOTH` leads to an error.
+>
+{type="warning"}
 
 The compiler type can also be set in the `gradle.properties` file, with the key `kotlin.js.compiler=ir`.
 This behaviour is overwritten by any settings in the `build.gradle(.kts)`, however.