chore(deps): bump starlette from 1.2.1 to 1.3.1 in /backend

Bumps [starlette](https://github.com/Kludex/starlette) from 1.2.1 to 1.3.1.
- [Release notes](https://github.com/Kludex/starlette/releases)
- [Changelog](https://github.com/Kludex/starlette/blob/main/docs/release-notes.md)
- [Commits](Kludex/starlette@1.2.1...1.3.1)

---
updated-dependencies:
- dependency-name: starlette
  dependency-version: 1.3.1
  dependency-type: indirect
...

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

Author: dependabot[bot]