Prohibit JVM target 1.6

But still compile stdlib, reflect, kotlin.test and scripting runtimes
with JVM target 1.6 to simplify migration from Kotlin 1.6 to 1.7.

 #KT-45165 Fixed

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

@@ -37,7 +37,7 @@ open class DefaultValues(val defaultValue: String, val possibleValues: List<Stri
 
     object JvmTargetVersions : DefaultValues(
         "null",
-        JvmTarget.values().map { "\"${it.description}\"" }
+        JvmTarget.supportedValues().map { "\"${it.description}\"" }
     )
 
     object JsEcmaVersions : DefaultValues(

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

@@ -75,7 +75,7 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
     @Argument(
         value = "-jvm-target",
         valueDescription = "<version>",
-        description = "Target version of the generated JVM bytecode (1.6 (DEPRECATED), 1.8, 9, 10, ..., 18), default is 1.8"
+        description = "Target version of the generated JVM bytecode (1.8, 9, 10, ..., 18), default is 1.8"
     )
     var jvmTarget: String? by NullableStringFreezableVar(null)
 
@@ -466,7 +466,8 @@ Also sets `-jvm-target` value equal to the selected JDK version"""
 
     @Argument(
         value = "-Xsuppress-deprecated-jvm-target-warning",
-        description = "Suppress deprecation warning about deprecated JVM target versions"
+        description = "Suppress deprecation warning about deprecated JVM target versions.\n" +
+                "This option has no effect and will be deleted in a future version."
     )
     var suppressDeprecatedJvmTargetWarning: Boolean by FreezableVar(false)
 

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

@@ -56,10 +56,10 @@ fun CompilerConfiguration.setupJvmSpecificArguments(arguments: K2JVMCompilerArgu
         val jvmTarget = JvmTarget.fromString(jvmTargetValue)
         if (jvmTarget != null) {
             put(JVMConfigurationKeys.JVM_TARGET, jvmTarget)
-            if (jvmTarget == JvmTarget.JVM_1_6 && !arguments.suppressDeprecatedJvmTargetWarning) {
+            if (jvmTarget == JvmTarget.JVM_1_6 && !isJvmTarget6Allowed()) {
                 messageCollector.report(
-                    STRONG_WARNING,
-                    "JVM target 1.6 is deprecated and will be removed in a future release. Please migrate to JVM target 1.8 or above"
+                    ERROR,
+                    "JVM target 1.6 is no longer supported. Please migrate to JVM target 1.8 or above"
                 )
             }
         } else {
@@ -353,3 +353,6 @@ private val CompilerConfiguration.messageCollector: MessageCollector
 
 private fun getJavaVersion(): Int =
     System.getProperty("java.specification.version")?.substringAfter('.')?.toIntOrNull() ?: 6
+
+private fun isJvmTarget6Allowed(): Boolean =
+    K2JVMCompiler::class.java.classLoader.getResource("META-INF/unsafe-allow-jvm6") != null