chore(deps): bump ash from 2.17.1 to 2.21.15 in /backend-elixir

Bumps [ash](https://github.com/ash-project/ash) from 2.17.1 to 2.21.15.
- [Changelog](https://github.com/ash-project/ash/blob/v2.21.15/CHANGELOG.md)
- [Commits](ash-project/ash@v2.17.1...v2.21.15)

---
updated-dependencies:
- dependency-name: ash
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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