Schedule maintenance window over "Free" events. (#21269)

#21268 
# Checklist for submitter

If some of the following don't apply, delete the relevant line.

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- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/Committing-Changes.md#changes-files)
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- [x] Manual QA for all new/changed functionality

changes/21268-calendar-free

@@ -0,0 +1 @@
+Scheduled maintenance events will now be scheduled over calendar events marked "Free" (not busy) in Google Calendar.

ee/server/calendar/google_calendar.go

@@ -597,6 +597,11 @@ func (c *GoogleCalendar) createEvent(
 			continue
 		}
 
+		if gEvent.Transparency == "transparent" {
+			// Ignore events that do not block time on the calendar (e.g. free events)
+			continue
+		}
+
 		// Ignore events that the user has declined
 		var declined bool
 		for _, attendee := range gEvent.Attendees {