Bump gatsby from 2.15.34 to 2.21.19

Bumps [gatsby](https://github.com/gatsbyjs/gatsby) from 2.15.34 to 2.21.19.
- [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/[email protected]@2.21.19)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

package.json

@@ -6,7 +6,7 @@
   "author": "Python Pizza",
   "dependencies": {
     "classnames": "^2.2.6",
-    "gatsby": "^2.15.34",
+    "gatsby": "^2.21.19",
     "gatsby-image": "^2.2.23",
     "gatsby-plugin-manifest": "^2.2.20",
     "gatsby-plugin-offline": "^3.0.13",