Bump starlette in /backend in the pip group across 1 directory

Bumps the pip group with 1 update in the /backend directory: [starlette](https://github.com/encode/starlette).


Updates `starlette` from 0.38.5 to 0.40.0
- [Release notes](https://github.com/encode/starlette/releases)
- [Changelog](https://github.com/encode/starlette/blob/master/docs/release-notes.md)
- [Commits](Kludex/starlette@0.38.5...0.40.0)

---
updated-dependencies:
- dependency-name: starlette
  dependency-type: indirect
  dependency-group: pip
...

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

Author: dependabot[bot]