Bump github.com/docker/distribution

Bumps [github.com/docker/distribution](https://github.com/docker/distribution) from 2.7.1+incompatible to 2.8.0+incompatible.
- [Release notes](https://github.com/docker/distribution/releases)
- [Commits](https://github.com/docker/distribution/compare/v2.7.1...v2.8.0)

---
updated-dependencies:
- dependency-name: github.com/docker/distribution
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

vendor/github.com/docker/distribution/registry/api/errcode/errors.go

@@ -207,11 +207,11 @@ func (errs Errors) MarshalJSON() ([]byte, error) {
 	for _, daErr := range errs {
 		var err Error
 
-		switch daErr.(type) {
+		switch daErr := daErr.(type) {
 		case ErrorCode:
-			err = daErr.(ErrorCode).WithDetail(nil)
+			err = daErr.WithDetail(nil)
 		case Error:
-			err = daErr.(Error)
+			err = daErr
 		default:
 			err = ErrorCodeUnknown.WithDetail(daErr)