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

compiler/fir/modularized-tests/tests/org/jetbrains/kotlin/fir/FE1FullPipelineModularizedTest.kt

@@ -15,7 +15,7 @@ class FE1FullPipelineModularizedTest : AbstractFullPipelineModularizedTest() {
     override fun configureArguments(args: K2JVMCompilerArguments, moduleData: ModuleData) {
         args.useIR = USE_BE_IR
         args.useOldBackend = !USE_BE_IR
-        args.useFir = false
+        args.useK2 = false
         args.jvmDefault = "compatibility"
         args.apiVersion = "1.4"
         args.optIn = arrayOf(

compiler/fir/modularized-tests/tests/org/jetbrains/kotlin/fir/FullPipelineModularizedTest.kt

@@ -12,7 +12,7 @@ private val LANGUAGE_VERSION: String = System.getProperty("fir.bench.language.ve
 class FullPipelineModularizedTest : AbstractFullPipelineModularizedTest() {
 
     override fun configureArguments(args: K2JVMCompilerArguments, moduleData: ModuleData) {
-        args.useFir = true
+        args.useK2 = true
         args.useIR = true
         args.apiVersion = LANGUAGE_VERSION
         args.jvmDefault = "compatibility"