replace add2calendar with add-to-calendar-button or similar

[compwron]

Since add2calendar is actively used and has 77 vulnerabilities (including 9 critical), this is a
high-priority security issue that should be addressed by replacing it with a modern alternative
like add-to-calendar-button.

[Budmin]

Hey! I'm looking to get involved and this seemed like a pretty easy one to start with.

Quick question, is this issue already being worked on? There's a draft for this and if this isn't actually available I'll move on to a different issue

[github-actions]

This issue has been open without changes for a long time! What's up?