Deprecate the `lang` XML namespace

The `lang` XML namespace (used with `<lang:groovy/>`, `<lang:bsh/>` and `<lang:std/>` for example) is not actively maintained anymore, there is no annotation-driven alternative and no intent to provide one. As a consequence, we deprecate this feature and related classes as of Spring Framework 7.0. The related "Dynamic Language Support" documentation will be removed.