** The default script language is now maintained in `Script` class.
* Added `script.legacy.default_lang` setting that controls the default language for scripts that are stored inside documents (for example percolator queries). This defaults to groovy.
** Added `QueryParseContext#getDefaultScriptLanguage()` that manages the default scripting language. Returns always `painless`, unless loading query/search request in legacy mode then the returns what is configured in `script.legacy.default_lang` setting.
** In the aggregation parsing code added `ParserContext` that also holds the default scripting language like `QueryParseContext`. Most parser don't have access to `QueryParseContext`. This is for scripts in aggregations.
* The `lang` script field is always serialized (toXContent).
Closes#20122