build(deps): bump gatsby-plugin-sitemap from 5.13.0 to 5.21.0 in /docs (#3169)

Author: dependabot[bot]

docs/package-lock.json

@@ -25,7 +25,7 @@
     "gatsby-plugin-react-svg": "^3.1.0",
     "gatsby-plugin-robots-txt": "^1.7.1",
     "gatsby-plugin-sharp": "^4.19.0",
-    "gatsby-plugin-sitemap": "^5.13.0",
+    "gatsby-plugin-sitemap": "^5.21.0",
     "gatsby-remark-autolink-headers": "^5.15.1",
     "gatsby-remark-copy-linked-files": "^5.21.0",
     "gatsby-remark-external-links": "^0.0.4",