build(deps-dev): bump starlette from 0.48.0 to 0.50.0 in /backend

Bumps [starlette](https://github.com/Kludex/starlette) from 0.48.0 to 0.50.0.
- [Release notes](https://github.com/Kludex/starlette/releases)
- [Changelog](https://github.com/Kludex/starlette/blob/main/docs/release-notes.md)
- [Commits](Kludex/starlette@0.48.0...0.50.0)

---
updated-dependencies:
- dependency-name: starlette
  dependency-version: 0.50.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]