Bump highlight.js from 9.13.1 to 9.15.6 in /slides

Bumps [highlight.js](https://github.com/highlightjs/highlight.js) from 9.13.1 to 9.15.6.
- [Release notes](https://github.com/highlightjs/highlight.js/releases)
- [Changelog](https://github.com/highlightjs/highlight.js/blob/master/CHANGES.md)
- [Commits](highlightjs/highlight.js@9.13.1...9.15.6)

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

Author: dependabot-support

slides/package-lock.json

@@ -3,7 +3,7 @@
   "author": "Artem Malyshev",
   "private": true,
   "dependencies": {
-    "highlight.js": "^9.12.0",
+    "highlight.js": "^9.15.6",
     "reveal.js": "^3.7.0"
   },
   "devDependencies": {