Bump postcss and gatsby

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [gatsby](https://github.com/gatsbyjs/gatsby). These dependencies need to be updated together.


Updates `postcss` from 6.0.23 to 8.4.31
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/6.0.23...8.4.31)

Updates `gatsby` from 2.17.6 to 5.12.6
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/compare/gatsby@2.17.6...gatsby@5.12.6)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: gatsby
  dependency-type: direct:production
...

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

package.json

@@ -5,7 +5,7 @@
   "version": "0.1.0",
   "author": "Kyle Mathews <mathews.kyle@gmail.com>",
   "dependencies": {
-    "gatsby": "^2.17.6",
+    "gatsby": "^5.12.7",
     "gatsby-image": "^2.2.30",
     "gatsby-plugin-manifest": "^2.2.25",
     "gatsby-plugin-offline": "^3.0.17",