Deprecate JVM target 1.6

#KT-44650 Fixed

compiler/testData/cli/jvm/extraHelp.out

@@ -118,6 +118,8 @@ where advanced options include:
   -Xsupport-compatqual-checker-framework-annotations=enable|disable
                              Specify behavior for Checker Framework compatqual annotations (NullableDecl/NonNullDecl).
                              Default value is 'enable'
+  -Xsuppress-deprecated-jvm-target-warning
+                             Suppress deprecation warning about deprecated JVM target versions
   -Xsuppress-missing-builtins-error
                              Suppress the "cannot access built-in declaration" error (useful with -no-stdlib)
   -Xuse-ir                   Use the IR backend

compiler/testData/cli/jvm/help.out

@@ -6,7 +6,7 @@ where possible options include:
   -include-runtime           Include Kotlin runtime into the resulting JAR
   -java-parameters           Generate metadata for Java 1.8 reflection on method parameters
   -jdk-home <path>           Include a custom JDK from the specified location into the classpath instead of the default JAVA_HOME
-  -jvm-target <version>      Target version of the generated JVM bytecode (1.6, 1.8, 9, 10, 11, 12, 13, 14 or 15), default is 1.8
+  -jvm-target <version>      Target version of the generated JVM bytecode (1.6 (DEPRECATED), 1.8, 9, 10, 11, 12, 13, 14 or 15), default is 1.8
   -module-name <name>        Name of the generated .kotlin_module file
   -no-jdk                    Don't automatically include the Java runtime into the classpath
   -no-reflect                Don't automatically include Kotlin reflection into the classpath

compiler/testData/cli/jvm/jvm6Target.args

@@ -0,0 +1,5 @@
+$TESTDATA_DIR$/jvm8Target.kt
+-d
+$TEMP_DIR$
+-jvm-target
+1.6

compiler/testData/cli/jvm/jvm6Target.out

@@ -0,0 +1,2 @@
+warning: JVM target 1.6 is deprecated and will be removed in a future release. Please migrate to JVM target 1.8 or above
+OK

compiler/testData/cli/jvm/jvmDefaultAll.out

@@ -1,2 +1,3 @@
+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: '-Xjvm-default=all' is only supported since JVM target 1.8. Recompile with '-jvm-target 1.8'
 COMPILATION_ERROR