Enable runtime string concatenation by default (for -Xjvm-target=9 or greater)

#KT-42522 Fixed

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

@@ -377,7 +377,7 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
 default: `indy-with-constants` for JVM target 9 or greater, `inline` otherwise"""
 
     )
-    var stringConcat: String? by NullableStringFreezableVar(JvmStringConcat.INLINE.description)
+    var stringConcat: String? by NullableStringFreezableVar(null)
 
     @Argument(
         value = "-Xsam-conversions",

compiler/cli/src/org/jetbrains/kotlin/cli/jvm/jvmArguments.kt

@@ -54,19 +54,20 @@ fun CompilerConfiguration.setupJvmSpecificArguments(arguments: K2JVMCompilerArgu
         }
     }
 
-    if (arguments.stringConcat != null) {
-        val runtimeStringConcat = JvmStringConcat.fromString(arguments.stringConcat!!)
+    val stringConcat = arguments.stringConcat
+    if (stringConcat != null) {
+        val runtimeStringConcat = JvmStringConcat.fromString(stringConcat)
         if (runtimeStringConcat != null) {
             put(JVMConfigurationKeys.STRING_CONCAT, runtimeStringConcat)
             if (jvmTarget.majorVersion < JvmTarget.JVM_9.majorVersion && runtimeStringConcat != JvmStringConcat.INLINE) {
                 messageCollector.report(
                     WARNING,
-                    "`-Xstring-concat=${arguments.stringConcat}` does nothing with JVM target `${jvmTarget.description}`."
+                    "`-Xstring-concat=$stringConcat` does nothing with JVM target `${jvmTarget.description}`."
                 )
             }
         } else {
             messageCollector.report(
-                ERROR, "Unknown `-Xstring-concat` mode: ${arguments.stringConcat}\n" +
+                ERROR, "Unknown `-Xstring-concat` mode: $stringConcat\n" +
                         "Supported modes: ${JvmStringConcat.values().joinToString { it.description }}"
             )
         }

compiler/testData/codegen/bytecodeText/stringOperations/concatDynamic.kt

@@ -1,4 +1,3 @@
-// KOTLIN_CONFIGURATION_FLAGS: STRING_CONCAT=indy-with-constants
 // JVM_TARGET: 9
 class A