fix: upgrade gatsby to 4.21.0 to fix Firefox dev

on `gatsby` 4.19.0, Firefox won't render the site in development
if there are any runtime errors. this was fixed in 4.21.0.
see gatsbyjs/gatsby#36365

Author: sunnyzanchi

package.json

@@ -15,7 +15,7 @@
     "common-tags": "^1.8.0",
     "date-fns": "^2.17.0",
     "front-matter": "^4.0.2",
-    "gatsby": "~4.19.0",
+    "gatsby": "4.21.0",
     "gatsby-image": "^3.11.0",
     "gatsby-plugin-gatsby-cloud": "~4.19.0",
     "gatsby-plugin-json-output": "^1.2.0",