Gatsby 5.16.1, node 24, deps updates and cleanups

[fturmel]

A bit of modernizing and patching deps. It's unfortunate that Gatsby is abandonware at this point, but at least they are supporting the active Node LTS.

Deps updates

• Gatsby updated to latest (5.16.1) https://www.gatsbyjs.com/docs/reference/release-notes/v5.16/
• Node 24 (current LTS) https://nodejs.org/en/about/previous-releases
• React 19 and most deps upgraded to latest
  • mdx related deps need to stay at their current major version to ensure compat with gatsby-plugin-mdx

Cleanups

• Replaced React Helmet usage with Gatsby Head API https://www.gatsbyjs.com/docs/reference/built-in-components/gatsby-head/
• Replaced prop-types with JSDocs
• Moved css modules in ./src/styles/pages to src/pages so they are colocated with their component files just like the rest of the project
• Github Actions versions updated to latest, and Node versions reads from nvmrc file directly
• Removed gatsby-plugin-netlify. Since Gatsby 5.12, gatsby-adapter-netlify is automatically configured during deployment. Having gatsby-adapter-netlify in our package.json deps will help speed up deployments a bit. https://docs.netlify.com/build/frameworks/framework-setup-guides/gatsby/#netlify-integration

Hopefully nothing broke in the process. Everything looked good locally.

EDIT 1:

• Build failures were related to previous build caches being incompatible. If merged, we will most likely need to manually clear the Netlify cache for the main branch.

EDIT 2:

• There were hydration errors in the console with React 19 production builds, I'm not sure that Gatsby's compatibility claims were completely accurate. We should stay on 18.

EDIT 3:

• Removed some dead code/CSS and unused exports.
• Removed unused google-auth-library dep.
• Improved filters clearing on all pages. It used to keep "all" in the URL, but now just strips the filters from the URL when every filter on a page has been cleared. Affects all pages with filters (challenges, tracks, showcases).

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