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/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