Bump prismjs from 1.15.0 to 1.21.0

Bumps [prismjs](https://github.com/PrismJS/prism) from 1.15.0 to 1.21.0.
- [Release notes](https://github.com/PrismJS/prism/releases)
- [Changelog](https://github.com/PrismJS/prism/blob/master/CHANGELOG.md)
- [Commits](PrismJS/prism@v1.15.0...v1.21.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

package.json

@@ -43,7 +43,7 @@
     "material-ui-dropzone": "^3.5.0",
     "medium-editor": "^5.23.3",
     "polished": "^2.3.0",
-    "prismjs": "^1.15.0",
+    "prismjs": "^1.21.0",
     "prop-types": "^15.6.2",
     "ptz-i18n": "^1.0.0",
     "qs": "^6.9.4",