build(deps): bump gatsby-source-medium in /gatsby-theme-mate

Bumps [gatsby-source-medium](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-source-medium) from 4.25.0 to 5.12.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-source-medium/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-source-medium@5.12.0/packages/gatsby-source-medium)

---
updated-dependencies:
- dependency-name: gatsby-source-medium
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

gatsby-theme-mate/package.json

@@ -44,7 +44,7 @@
     "gatsby-plugin-styled-components": "^5.10.0",
     "gatsby-plugin-typescript": "^4.10.0",
     "gatsby-source-contentful": "^7.8.1",
-    "gatsby-source-medium": "^4.10.0",
+    "gatsby-source-medium": "^5.12.0",
     "gatsby-transformer-remark": "^5.25.1",
     "react-awesome-reveal": "^4.2.7",
     "react-headroom": "^3.2.1",