Update icalendar requirement from ~=5.0.11 to ~=6.0.1

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@v5.0.11...v6.0.1)

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

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

src/requirements.txt

@@ -7,7 +7,7 @@ requests ~= 2.32
 PyPDF4 ~= 1.27
 waitress ~= 2.1
 beautifulsoup4 ~= 4.12
-icalendar ~= 5.0.12
+icalendar ~= 6.0.1
 pytz ~= 2023.3
 werkzeug ~= 3.0.3
 bugsnag[flask] ~= 4.7.1