Maintenance: Update dependency icalendar to v2.9.0

Gemfile.lock

@@ -252,7 +252,7 @@ GEM
     httpclient (2.8.3)
     i18n (1.14.1)
       concurrent-ruby (~> 1.0)
-    icalendar (2.8.0)
+    icalendar (2.9.0)
       ice_cube (~> 0.16)
     icalendar-recurrence (1.1.3)
       icalendar (~> 2.0)