build(deps): bump github.com/opencontainers/image-spec (#2214)

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

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go.mod

@@ -21,7 +21,7 @@ require (
 	github.com/mattn/go-isatty v0.0.20
 	github.com/moby/buildkit v0.12.5
 	github.com/moby/patternmatcher v0.6.0
-	github.com/opencontainers/image-spec v1.1.0-rc3
+	github.com/opencontainers/image-spec v1.1.0-rc6
 	github.com/opencontainers/selinux v1.11.0
 	github.com/pkg/errors v0.9.1
 	github.com/rhysd/actionlint v1.6.26