Bump jszip and grunt-zip in /slides

Bumps [jszip](https://github.com/Stuk/jszip) to 2.7.0 and updates ancestor dependency [grunt-zip](https://github.com/twolfson/grunt-zip). These dependencies need to be updated together.


Updates `jszip` from 2.5.0 to 2.7.0
- [Release notes](https://github.com/Stuk/jszip/releases)
- [Changelog](https://github.com/Stuk/jszip/blob/main/CHANGES.md)
- [Commits](https://github.com/Stuk/jszip/commits)

Updates `grunt-zip` from 0.17.1 to 0.20.0
- [Release notes](https://github.com/twolfson/grunt-zip/releases)
- [Changelog](https://github.com/twolfson/grunt-zip/blob/master/CHANGELOG.md)
- [Commits](twolfson/grunt-zip@0.17.1...0.20.0)

---
updated-dependencies:
- dependency-name: jszip
  dependency-type: indirect
- dependency-name: grunt-zip
  dependency-type: direct:development
...

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

Author: dependabot[bot]

slides/package-lock.json

@@ -34,7 +34,7 @@
     "grunt-contrib-uglify": "^3.3.0",
     "grunt-contrib-watch": "^1.1.0",
     "grunt-sass": "^3.0.2",
-    "grunt-zip": "~0.17.1",
+    "grunt-zip": "~0.20.0",
     "load-grunt-tasks": "^4.0.0",
     "mustache": "^2.3.0",
     "node-sass": "^4.13.1",