Add JVM target bytecode version 21

Unfortunately, there are still problems with running JVM backend tests
with JVM target 21 because:

1) The D8 version that we use does not support bytecode version 21, and
   updating to a newer version requires to change JVM target of compiler
   tests to 11. This will be addressed separately when we enable JVM 21
   in tests-different-jdk.
2) Some tests are failing because of KT-60659 and KT-60770:
   - builtinStubMethods/bridgesForStubs/emptyStringListAdd.kt
   - jdk/stream.kt
   - regressions/kt528.kt

 #KT-60662 Fixed

compiler/testData/cli/jvm/extraHelp.out

@@ -42,7 +42,7 @@ where advanced options include:
   -Xjava-source-roots=<path> Paths to directories with Java source files
   -Xjavac-arguments=<option[,]> Java compiler arguments
   -Xjdk-release=<version>    Compile against the specified JDK API version, similarly to javac's `-release`. Requires JDK 9 or newer.
-                             Supported versions depend on the used JDK; for JDK 17+ supported versions are 1.8, 9, 10, ..., 20.
+                             Supported versions depend on the used JDK; for JDK 17+ supported versions are 1.8, 9, 10, ..., 21.
                              Also sets `-jvm-target` value equal to the selected JDK version
   -Xjspecify-annotations=ignore|strict|warn
                              Specify behavior for jspecify annotations.