Remove the Dutch Web Guidelines validation

The flavor web standards tests validate the pages of the flavor against the Dutch Web Guidelines (DWG), an old Dutch accessibility standard. Our accessibility commitment is WCAG 2.2 AA, which is validated by axe-core in the Docker functional tests, and the DWG checks are either not WCAG requirements, AAA-only, obsolete, or already covered by axe-core or the HTML5 (Nu) validator.

Agreed in https://forum.xwiki.org/t/remove-the-dutch-web-guidelines-validation/19056

• Remove the DWG validation from the flavor web standards tests.
• Remove HTML5DutchWebGuidelinesValidator from xwiki-platform-tool-standards-validator.