Deprecate JVM target 1.6

#KT-44650 Fixed

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

@@ -71,7 +71,7 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
     @Argument(
         value = "-jvm-target",
         valueDescription = "<version>",
-        description = "Target version of the generated JVM bytecode (1.6, 1.8, 9, 10, 11, 12, 13, 14 or 15), default is 1.8"
+        description = "Target version of the generated JVM bytecode (1.6 (DEPRECATED), 1.8, 9, 10, 11, 12, 13, 14 or 15), default is 1.8"
     )
     var jvmTarget: String? by NullableStringFreezableVar(JvmTarget.DEFAULT.description)
 
@@ -448,6 +448,12 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
     )
     var enableJvmPreview: Boolean by FreezableVar(false)
 
+    @Argument(
+        value = "-Xsuppress-deprecated-jvm-target-warning",
+        description = "Suppress deprecation warning about deprecated JVM target versions"
+    )
+    var suppressDeprecatedJvmTargetWarning: Boolean by FreezableVar(false)
+
     override fun configureAnalysisFlags(collector: MessageCollector): MutableMap<AnalysisFlag<*>, Any> {
         val result = super.configureAnalysisFlags(collector)
         result[JvmAnalysisFlags.strictMetadataVersionSemantics] = strictMetadataVersionSemantics

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

@@ -29,6 +29,12 @@ fun CompilerConfiguration.setupJvmSpecificArguments(arguments: K2JVMCompilerArgu
         val jvmTarget = JvmTarget.fromString(arguments.jvmTarget!!)
         if (jvmTarget != null) {
             put(JVMConfigurationKeys.JVM_TARGET, jvmTarget)
+            if (jvmTarget == JvmTarget.JVM_1_6 && !arguments.suppressDeprecatedJvmTargetWarning) {
+                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"
+                )
+            }
         } else {
             messageCollector.report(
                 ERROR, "Unknown JVM target version: ${arguments.jvmTarget}\n" +