chore(deps): bump the gatsby group with 2 updates

Bumps the gatsby group with 2 updates: [gatsby](https://github.com/gatsbyjs/gatsby) and [gatsby-plugin-sitemap](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-sitemap).


Updates `gatsby` from 5.11.0 to 5.12.3
- [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/[email protected]@5.12.3)

Updates `gatsby-plugin-sitemap` from 6.11.0 to 6.12.0
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-sitemap/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/[email protected]/packages/gatsby-plugin-sitemap)

---
updated-dependencies:
- dependency-name: gatsby
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: gatsby
- dependency-name: gatsby-plugin-sitemap
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: gatsby
...

Signed-off-by: dependabot[bot] <[email protected]>

package.json

@@ -13,9 +13,9 @@
   "dependencies": {
     "@thepolicylab-projectportals/gatsby-theme-project-portal": "^7.0.0",
     "@thepolicylab-projectportals/project-portal-content-netlify": "^7.0.0",
-    "gatsby": "^5.11.0",
+    "gatsby": "^5.12.3",
     "gatsby-plugin-netlify": "^5.1.0",
-    "gatsby-plugin-sitemap": "^6.11.0",
+    "gatsby-plugin-sitemap": "^6.12.0",
     "react": "^18.2.0",
     "react-dom": "^18.2.0"
   },