Use TryGetValue instead of ContainsKey + index access to avoid double…

… lookup (#193)

This PR fixes an issue that is currently causing CI to fail in all pull
requests:
`Notice: "[CA1854] Prefer a 'TryGetValue' call over a Dictionary indexer
access guarded by a 'ContainsKey' check to avoid double lookup" on
/home/runner/work/Octobot/Octobot/src/Services/Update/ScheduledEventUpdateService.cs(107,4168)`
The issue is resolved by following the advice mentioned in the notice.

src/Services/Update/ScheduledEventUpdateService.cs

@@ -104,14 +104,13 @@ private static void SyncScheduledEvents(GuildData data, IEnumerable<IGuildSchedu
     {
         foreach (var @event in events)
         {
-            if (!data.ScheduledEvents.ContainsKey(@event.ID.Value))
+            if (!data.ScheduledEvents.TryGetValue(@event.ID.Value, out var eventData))
             {
                 data.ScheduledEvents.Add(@event.ID.Value,
                     new ScheduledEventData(@event.ID.Value, @event.Name, @event.ScheduledStartTime, @event.Status));
                 continue;
             }
 
-            var eventData = data.ScheduledEvents[@event.ID.Value];
             eventData.Name = @event.Name;
             eventData.ScheduledStartTime = @event.ScheduledStartTime;
             if (!eventData.ScheduleOnStatusUpdated)