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/app/post_acknowledgements_test.go

@@ -52,7 +52,7 @@ func testSaveAcknowledgementForPost(t *testing.T) {
 		_, err = th.App.SaveAcknowledgementForPost(th.Context, post.Id, th.BasicUser.Id)
 		require.Nil(t, err)
 
-		post, err = th.App.GetSinglePost(post.Id, false)
+		post, err = th.App.GetSinglePost(th.Context, post.Id, false)
 		require.Nil(t, err)
 
 		require.Greater(t, post.UpdateAt, oldUpdateAt)
@@ -91,7 +91,7 @@ func testDeleteAcknowledgementForPost(t *testing.T) {
 		_, err := th.App.SaveAcknowledgementForPost(th.Context, post.Id, th.BasicUser.Id)
 		require.Nil(t, err)
 
-		post, err = th.App.GetSinglePost(post.Id, false)
+		post, err = th.App.GetSinglePost(th.Context, post.Id, false)
 		require.Nil(t, err)
 
 		oldUpdateAt := post.UpdateAt
@@ -99,7 +99,7 @@ func testDeleteAcknowledgementForPost(t *testing.T) {
 		err = th.App.DeleteAcknowledgementForPost(th.Context, post.Id, th.BasicUser.Id)
 		require.Nil(t, err)
 
-		post, err = th.App.GetSinglePost(post.Id, false)
+		post, err = th.App.GetSinglePost(th.Context, post.Id, false)
 		require.Nil(t, err)
 
 		require.Greater(t, post.UpdateAt, oldUpdateAt)