Bump starlette from 0.27.0 to 0.40.0 in /backend

Bumps [starlette](https://github.com/encode/starlette) from 0.27.0 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](encode/starlette@0.27.0...0.40.0)

---
updated-dependencies:
- dependency-name: starlette
  dependency-type: direct:production
...

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

backend/requirements.txt

@@ -36,7 +36,7 @@ six==1.16.0
 sh==1.14.3
 # SQLAlchemy==2.0.20
 SQLAlchemy==1.4.41
-starlette==0.27.0
+starlette==0.40.0
 # sse-starlette==1.4.1 (wrong version cant deploy)
 sse-starlette==2.1.3
 typing-extensions==4.7.1