Bump gatsby from 2.23.7 to 2.23.21

Bumps [gatsby](https://github.com/gatsbyjs/gatsby) from 2.23.7 to 2.23.21.
- [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/gatsby@2.23.7...gatsby@2.23.21)

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

package.json

@@ -7,7 +7,7 @@
     "axios": "^0.19.2",
     "bytes": "^3.1.0",
     "classnames": "^2.2.6",
-    "gatsby": "^2.23.7",
+    "gatsby": "^2.23.21",
     "gatsby-image": "^2.4.12",
     "gatsby-plugin-manifest": "^2.4.17",
     "gatsby-plugin-matomo": "^0.8.3",