Update icalendar requirement from ==5.0.* to ==6.1.* in /server (#280)

Updates the requirements on [icalendar](https://github.com/collective/icalendar) to permit the latest version.
- [Release notes](https://github.com/collective/icalendar/releases)
- [Changelog](https://github.com/collective/icalendar/blob/main/CHANGES.rst)
- [Commits](collective/icalendar@5.0.0a1...v6.1.0)

---
updated-dependencies:
- dependency-name: icalendar
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

server/requirements.txt

@@ -13,7 +13,7 @@ django-formset-js-improved
 django-multifactor==0.7.2
 djangorestframework==3.15.*
 emoji==2.14.*
-icalendar==5.0.*
+icalendar==6.1.*
 lxml==5.3.*
 matplotlib==3.9.*
 numpy==2.1.*