chore(deps): bump ngx-markdown from 8.2.1 to 9.1.1 in /frontend

Bumps [ngx-markdown](https://github.com/jfcere/ngx-markdown) from 8.2.1 to 9.1.1.
- [Release notes](https://github.com/jfcere/ngx-markdown/releases)
- [Commits](jfcere/ngx-markdown@v8.2.1...v9.1.1)

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

frontend/package.json

@@ -28,7 +28,7 @@
     "@fortawesome/free-solid-svg-icons": "^5.11.2",
     "@ng-select/ng-select": "^4.0.0",
     "core-js": "^3.6.5",
-    "ngx-markdown": "^8.2.1",
+    "ngx-markdown": "^9.1.1",
     "rxjs": "~6.5.4",
     "tslib": "^1.10.0",
     "typescript": "^3.8.3",