Incorrect "PR #5 remains open" remains open after git stack modify for a PR that is already closed

[harsh183]

1. Use gs submit to create stack with depth 2. Let's say branch names layer1 and layer2
2. I close the PR for layer2 in the GitHub UI
3. I want to remove the stack locally, so I use git stack modify, press x to delete the layer, and then Control+S to save.

I get the following output

$ gs modify
✓ Dropped layer2 from stack

✓ Stack modified successfully
  Dropped: layer 2(PR #53155 remains open — close with `gh pr close 53155`)

Run `gh stack submit` to push your changes and update the stack of PRs on GitHub

Now gh cli can clearly tell the PR is already closed, because when I try to close it:

$ gh pr close 53155
! Pull request repo#53155 (PR Title) is already closed

I think a fix will be doing a resync on the state from GitHub before showing this message.