fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-POSTCSS-1255640

Author: snyk-bot

package.json

@@ -22,7 +22,7 @@
 		"debug": "^4.3.1",
 		"documentation": "^13.2.0",
 		"fs-extra": "^7.0.1",
-		"gatsby": "^2.32.11",
+		"gatsby": "^3.0.0",
 		"gatsby-plugin-advanced-sitemap": "^1.6.0",
 		"gatsby-plugin-react-helmet": "^3.3.3",
 		"gatsby-plugin-sass": "^2.8.0",
@@ -39,7 +39,7 @@
 		"moment-strftime": "^0.5.0",
 		"node-sass": "^4.14.1",
 		"node-sass-utils": "^1.1.2",
-		"postcss": "^8.2.8",
+		"postcss": "^8.2.13",
 		"react": "^16.14.0",
 		"react-dom": "^16.8.1",
 		"react-helmet": "^5.2.0",