JavaScript code is still transpiled to support IE11

This is a leftover from XWIKI-19327. There are two problems:

• I configured Closure Compiler to use STABLE for both language in and out but it seems that STABLE has a different meaning when applied to language in versus language out, see https://github.com/google/closure-compiler/issues/3679#issuecomment-689925089 . So in the end Closure Compiler is still transpiling the code to ES5_STRICT (IE11)
• I forgot to update the configuration used in JsExtension (Skinx) when minifying code from JSX objects.