Bump gatsby-plugin-sitemap from 2.0.11 to 2.2.20

Bumps [gatsby-plugin-sitemap](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-sitemap) from 2.0.11 to 2.2.20.
- [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@2.2.20/packages/gatsby-plugin-sitemap)

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

Author: dependabot-preview[bot]

package-lock.json

@@ -15,7 +15,7 @@
     "gatsby-plugin-offline": "^2.0.25",
     "gatsby-plugin-react-helmet": "^3.0.11",
     "gatsby-plugin-sharp": "^2.0.32",
-    "gatsby-plugin-sitemap": "^2.0.11",
+    "gatsby-plugin-sitemap": "^2.2.20",
     "gatsby-plugin-styled-components": "^3.1.11",
     "gatsby-source-filesystem": "^2.1.33",
     "gatsby-transformer-sharp": "^2.1.17",