fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-SANITIZEHTML-6256334

Author: snyk-bot

package.json

@@ -21,7 +21,7 @@
     "gatsby-remark-copy-linked-files": "^2.0.7",
     "gatsby-remark-images": "^3.0.1",
     "gatsby-source-filesystem": "^2.0.11",
-    "gatsby-transformer-remark": "^2.1.15",
+    "gatsby-transformer-remark": "^6.0.0",
     "gatsby-transformer-sharp": "^2.1.9",
     "lodash": "^4.17.4",
     "node-sass": "^4.9.3",