Bump league/oauth2-client from 2.7.0 to 2.8.0 in /graph-tutorial

Bumps [league/oauth2-client](https://github.com/thephpleague/oauth2-client) from 2.7.0 to 2.8.0.
- [Release notes](https://github.com/thephpleague/oauth2-client/releases)
- [Changelog](https://github.com/thephpleague/oauth2-client/blob/master/CHANGELOG.md)
- [Commits](thephpleague/oauth2-client@2.7.0...2.8.0)

---
updated-dependencies:
- dependency-name: league/oauth2-client
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]