Update fastapi requirement from 0.115.5 to 0.115.6 in /backend

Updates the requirements on [fastapi](https://github.com/fastapi/fastapi) to permit the latest version.
- [Release notes](https://github.com/fastapi/fastapi/releases)
- [Commits](fastapi/fastapi@0.115.5...0.115.6)

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

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

backend/Pipfile

@@ -12,7 +12,7 @@ alembic = ">=1.13.1"
 bcrypt = "4.2.1"
 click = ">=8.1.3"
 databases = {extras = ["asyncpg"], version = "<=0.8.0"}
-fastapi = "0.115.5"
+fastapi = "0.115.6"
 fastapi-cache2 = ">=0.2.0"
 fastapi-sso = ">=0.6.4"
 gunicorn = ">=20.1.0"