chore(deps): bump prismjs from 1.20.0 to 1.21.0

Bumps [prismjs](https://github.com/PrismJS/prism) from 1.20.0 to 1.21.0.
- [Release notes](https://github.com/PrismJS/prism/releases)
- [Changelog](https://github.com/PrismJS/prism/blob/master/CHANGELOG.md)
- [Commits](https://github.com/PrismJS/prism/compare/v1.20.0...v1.21.0)

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -75,7 +75,7 @@
     "netlify-cms-app": "^2.12.19",
     "node-sass": "^4.14.1",
     "normalize-scss": "^7.0.1",
-    "prismjs": "^1.20.0",
+    "prismjs": "^1.21.0",
     "react": "^16.13.1",
     "react-disqus-comments": "^1.4.0",
     "react-dom": "^16.13.1",