Bump gatsby-plugin-sitemap from 2.10.0 to 2.12.0

Bumps [gatsby-plugin-sitemap](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-sitemap) from 2.10.0 to 2.12.0.
- [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/gatsby-plugin-sitemap@2.12.0/packages/gatsby-plugin-sitemap)

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

package.json

@@ -76,7 +76,7 @@
     "gatsby-plugin-react-helmet": "^3.7.0",
     "gatsby-plugin-robots-txt": "^1.5.5",
     "gatsby-plugin-sharp": "^2.13.4",
-    "gatsby-plugin-sitemap": "^2.9.0",
+    "gatsby-plugin-sitemap": "^2.12.0",
     "gatsby-plugin-svgr": "^2.0.2",
     "gatsby-source-filesystem": "^2.8.1",
     "gatsby-transformer-sharp": "^2.11.0",