[Gradle, JS] IR by default and report warnings for using legacy compiler

- just legacy - report warning about deprecation
- both - report warning about deprecation of legacy
- no compiler explicitly chosen - error about explicit setting compiler

warn from cli legacy compiler

nowarn flag - kotlin.js.compiler.nowarn

KT-42326
KT-53074

compiler/tests/org/jetbrains/kotlin/cli/LauncherScriptTest.kt

@@ -141,7 +141,7 @@ class LauncherScriptTest : TestCaseWithTmpdir() {
         runProcess(
             "kotlinc-js",
             "$testDataDirectory/emptyMain.kt",
-            "-output", File(tmpdir, "out.js").path
+            "-Xlegacy-deprecated-no-warn", "-output", File(tmpdir, "out.js").path
         )
     }
 

compiler/tests/org/jetbrains/kotlin/jvm/compiler/CompileKotlinAgainstCustomBinariesTest.kt

@@ -122,7 +122,7 @@ class CompileKotlinAgainstCustomBinariesTest : AbstractKotlinCompilerIntegration
 
         val result =
             when (compiler) {
-                is K2JSCompiler -> compileJsLibrary(libraryName, additionalOptions = libraryOptions)
+                is K2JSCompiler -> compileJsLibrary(libraryName, additionalOptions = libraryOptions + "-Xlegacy-deprecated-no-warn")
                 is K2JVMCompiler -> compileLibrary(libraryName, additionalOptions = libraryOptions)
                 else -> throw UnsupportedOperationException(compiler.toString())
             }