MM-40818: Pass filestore to channels (#19677)

We move away from referencing server struct
but pass the filestore service to Channels
explicitly.

https://mattermost.atlassian.net/browse/MM-40818

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/file.go

@@ -48,7 +48,7 @@ const (
 )
 
 func (a *App) FileBackend() filestore.FileBackend {
-	return a.Srv().FileBackend()
+	return a.ch.filestore
 }
 
 func (a *App) CheckMandatoryS3Fields(settings *model.FileSettings) *model.AppError {