deps: update pytest-cov requirement from ^5.0.0 to ^6.0.0 in /backend

Updates the requirements on [pytest-cov](https://github.com/pytest-dev/pytest-cov) to permit the latest version.
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-cov@v5.0.0...v6.0.0)

---
updated-dependencies:
- dependency-name: pytest-cov
  dependency-type: direct:production
...

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

backend/pyproject.toml

@@ -21,7 +21,7 @@ pre-commit = "^4.0.1"
 alembic = { version = ">=1.11.1", optional = true }
 pytest = { version = ">=7.3.1", optional = true }
 httpx = { version = ">=0.24.0", optional = true }
-pytest-cov = { version = "^5.0.0", optional = true }
+pytest-cov = { version = "^6.0.0", optional = true }
 coverage = "^7.2.5"
 
 [tool.poetry.extras]