Update icalendar to version 2.12.3

Author: depfu[bot]

Gemfile.lock

@@ -247,7 +247,7 @@ GEM
     i18n (1.14.8)
       concurrent-ruby (~> 1.0)
     i18n_data (0.13.0)
-    icalendar (2.12.2)
+    icalendar (2.12.3)
       base64
       ice_cube (~> 0.16)
       logger