build(deps): bump openapi-backend from 5.20.1 to 5.20.3

Bumps [openapi-backend](https://github.com/openapistack/openapi-backend) from 5.20.1 to 5.20.3.
- [Release notes](https://github.com/openapistack/openapi-backend/releases)
- [Commits](openapistack/openapi-backend@5.20.1...5.20.3)

---
updated-dependencies:
- dependency-name: openapi-backend
  dependency-version: 5.20.3
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]