build(deps): bump @backstage/plugin-auth-backend from 0.18.3 to 0.18.8

Bumps [@backstage/plugin-auth-backend](https://github.com/backstage/backstage/tree/HEAD/plugins/auth-backend) from 0.18.3 to 0.18.8.
- [Release notes](https://github.com/backstage/backstage/releases)
- [Changelog](https://github.com/backstage/backstage/blob/master/plugins/auth-backend/CHANGELOG.md)
- [Commits](https://github.com/backstage/backstage/commits/HEAD/plugins/auth-backend)

---
updated-dependencies:
- dependency-name: "@backstage/plugin-auth-backend"
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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