[migration] Kotlin 1.9.20: KT-42326

There's no YT ticket for error-deprecating and/or removing legacy JS backend, so I'm linking the next closest thing.

Author: FenstonSingel

gradle.properties

@@ -13,7 +13,7 @@ java.mainToolchainVersion=8
 java.modularToolchainVersion=11
 
 kotlin.mpp.stability.nowarn=true
-kotlin.js.compiler=both
+kotlin.js.compiler=ir
 kotlin.native.ignoreIncorrectDependencies=true
 kotlin.native.ignoreDisabledTargets=true