[MM-63504] When changing a channel to group synced, it doesn't always clear members (#30526)

* fix issue with channel and team membership after group constraints are enabled

server/channels/api4/channel.go

@@ -391,6 +391,15 @@ func patchChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// If the channel is now group constrained but wasn't previously, delete members that aren't part of the channel's groups
+	if patch.GroupConstrained != nil && *patch.GroupConstrained && (originalOldChannel.GroupConstrained == nil || !*originalOldChannel.GroupConstrained) {
+		c.App.Srv().Go(func() {
+			if err := c.App.DeleteGroupConstrainedChannelMemberships(c.AppContext, &rchannel.Id); err != nil {
+				c.Logger.Warn("Error deleting group-constrained channel memberships", mlog.Err(err))
+			}
+		})
+	}
+
 	appErr = c.App.FillInChannelProps(c.AppContext, rchannel)
 	if appErr != nil {
 		c.Err = appErr