fix: www/package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-FLAT-596927
- https://snyk.io/vuln/npm:debug:20170905

www/package.json

@@ -8,7 +8,7 @@
         "start": "gatsby develop --port 8090"
     },
     "dependencies": {
-        "@mdx-js/mdx": "^1.6.5",
+        "@mdx-js/mdx": "^2.0.0",
         "@mdx-js/react": "^1.6.5",
         "@thumbtack/thumbprint-icons": "4.1.0",
         "@thumbtack/thumbprint-react": "^14.18.2",
@@ -17,7 +17,7 @@
         "clickable-box": "^1.0.0",
         "docsearch.js": "^2.6.2",
         "dotenv": "^8.1.0",
-        "gatsby": "^2.23.3",
+        "gatsby": "^5.9.0",
         "gatsby-plugin-compile-es6-packages": "^2.1.0",
         "gatsby-plugin-google-analytics": "^2.3.4",
         "gatsby-plugin-lodash": "^3.3.4",