MM-58038: Use context to call master for DeletePost (#27098)

Calling app.DeletePost immediately after creating a post
is susceptible to replica lag because we were calling the
replica to check for the post.

We fix this by passing a context to always query master.

https://mattermost.atlassian.net/browse/MM-58038

```release-note
NONE
```

server/channels/api4/preference.go

@@ -116,7 +116,7 @@ func updatePreferences(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	for _, pref := range preferences {
 		if pref.Category == model.PreferenceCategoryFlaggedPost {
-			post, err := c.App.GetSinglePost(pref.Name, false)
+			post, err := c.App.GetSinglePost(c.AppContext, pref.Name, false)
 			if err != nil {
 				c.SetInvalidParam("preference.name")
 				return