Update ical.js to unblock nextcloud/calendar-js

[kesselb]

Summary

To release the fix for nextcloud/calendar-js#790, calendar-js needs an update to v8.1.1.

#10286 fails with:

npm error code ERESOLVE
npm error ERESOLVE unable to resolve dependency tree
npm error
npm error While resolving: nextcloud-mail@4.3.0-alpha1
npm error Found: ical.js@1.5.0
npm error node_modules/ical.js
npm error   ical.js@"^1.5.0" from the root project
npm error
npm error Could not resolve dependency:
npm error peer ical.js@"^2.1.0" from @nextcloud/calendar-js@8.1.1
npm error node_modules/@nextcloud/calendar-js
npm error   @nextcloud/calendar-js@"^8.1.1" from the root project