Bump github.com/opencontainers/image-spec from 1.1.0-rc3 to 1.1.0-rc.6

Bumps [github.com/opencontainers/image-spec](https://github.com/opencontainers/image-spec) from 1.1.0-rc3 to 1.1.0-rc.6.
- [Release notes](https://github.com/opencontainers/image-spec/releases)
- [Changelog](https://github.com/opencontainers/image-spec/blob/main/RELEASES.md)
- [Commits](opencontainers/image-spec@v1.1.0-rc3...v1.1.0-rc6)

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

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