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/v2/urls.go

@@ -252,15 +252,3 @@ func appendValuesURL(u *url.URL, values ...url.Values) *url.URL {
 	u.RawQuery = merged.Encode()
 	return u
 }
-
-// appendValues appends the parameters to the url. Panics if the string is not
-// a url.
-func appendValues(u string, values ...url.Values) string {
-	up, err := url.Parse(u)
-
-	if err != nil {
-		panic(err) // should never happen
-	}
-
-	return appendValuesURL(up, values...).String()
-}