add support for Gatsby 5.x

[gdams]

Add support for Gatsby 5.x and deprecate the Helmet function (also deprecated upstream)

fixes: https://github.com/microapps/gatsby-plugin-react-i18next/issues/163

[jimmyn]

@gdams seems there is an issue with gatsby-plugin-sitemap

https://stackoverflow.com/questions/72250191/error-siteurl-does-not-exist-on-sitemetadata-in-the-data-returned-from-the

Could you please take a look?

[gdams]

@gdams seems there is an issue with gatsby-plugin-sitemap

Looking