Bump gatsby from 2.20.18 to 2.20.26

Bumps [gatsby](https://github.com/gatsbyjs/gatsby) from 2.20.18 to 2.20.26.
- [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@2.20.18...gatsby@2.20.26)

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

package.json

@@ -6,7 +6,7 @@
   "dependencies": {
     "axios": "^0.19.2",
     "classnames": "^2.2.6",
-    "gatsby": "^2.20.18",
+    "gatsby": "^2.20.26",
     "gatsby-image": "^2.3.3",
     "gatsby-plugin-manifest": "^2.3.3",
     "gatsby-plugin-matomo": "^0.8.3",