Bump gatsby-plugin-sitemap from 4.6.0 to 4.9.0

Bumps [gatsby-plugin-sitemap](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-sitemap) from 4.6.0 to 4.9.0.
- [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/gatsby-plugin-sitemap@4.9.0/packages/gatsby-plugin-sitemap)

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

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

package.json

@@ -77,7 +77,7 @@
     "gatsby-plugin-react-helmet": "^4.10.0",
     "gatsby-plugin-robots-txt": "^1.6.8",
     "gatsby-plugin-sharp": "^3.10.2",
-    "gatsby-plugin-sitemap": "^4.6.0",
+    "gatsby-plugin-sitemap": "^4.9.0",
     "gatsby-transformer-sharp": "^3.10.0",
     "github-syntax-light": "^0.5.0",
     "isomorphic-fetch": "^3.0.0",