Use Gatsby Head API

https://www.gatsbyjs.com/docs/reference/built-in-components/gatsby-head/

> Support for the Gatsby Head API was added in gatsby@4.19.0.
>
> Gatsby includes a built-in Head export that allows you to add elements to the [document head](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/head) of your pages.
>
> Compared to [react-helmet](https://github.com/nfl/react-helmet) or other similar solutions, Gatsby Head is easier to use, more performant, has a smaller bundle size, and supports the latest React features.

We'll have to change some things on the consumer websites as well, but our head tags have needed a refactor for a while so this may be a perfect opportunity!

Read the linked Gatsby Head API documentation first, then inspect how this theme currently provides head tags and which consumer websites use it. The work is done when the existing head-tag behavior is refactored to use Gatsby Head API and the affected consumer websites continue to receive the required tags.

Written by the indexing model from the issue text.