fix: add missing sign out page route for next.js app dir sample

[xiaohanyu]

The /api/logto/sign-out route is missing, this commit fix it, otherwise it will lead to a 404 not found.

Summary

The next.js app dir sample project is missing /api/logto/sign-out page route, and when user clicks sign-out button, it will lead to a 404 not found page.

Testing

Demo before fix: https://github.com/logto-io/js/assets/1164623/a4358f5f-5ce7-4fef-8d82-87cef49917eb

Checklist

• [x] ~.changeset~
• [x] ~unit tests~
• [x] ~integration tests~
• [x] ~necessary TSDoc comments~

[charIeszhao]

@xiaohanyu Hi, your PR is approved but our Git repo requires all commits to be signed in order to be merged into master. Could you please sign your commits?

[xiaohanyu]

commits signed.