Bump github.com/go-openapi/spec from 0.20.3 to 0.20.4 (#454)

Bumps [github.com/go-openapi/spec](https://github.com/go-openapi/spec) from 0.20.3 to 0.20.4.
- [Release notes](https://github.com/go-openapi/spec/releases)
- [Commits](https://github.com/go-openapi/spec/compare/v0.20.3...v0.20.4

)

---
updated-dependencies:
- dependency-name: github.com/go-openapi/spec
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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