Bump github.com/docker/distribution in /auth_server (cesanta#342)

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](distribution/distribution@v2.7.1...v2.8.0)

---
updated-dependencies:
- dependency-name: github.com/docker/distribution
  dependency-type: direct:production
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

auth_server/go.mod

@@ -7,19 +7,11 @@ require (
 	cloud.google.com/go/storage v1.14.0
 	github.com/casbin/casbin/v2 v2.24.0
 	github.com/cesanta/glog v0.0.0-20150527111657-22eb27a0ae19
-	github.com/cooldrip/cstrftime v0.0.0-20180425110708-e16e2f942e1e // indirect
 	github.com/coreos/go-oidc/v3 v3.0.0
 	github.com/dchest/uniuri v0.0.0-20200228104902-7aecb25e1fe5
 	github.com/deckarep/golang-set v1.7.1
-	github.com/docker/distribution v2.7.1+incompatible
+	github.com/docker/distribution v2.8.0+incompatible
 	github.com/docker/libtrust v0.0.0-20160708172513-aabc10ec26b7
-	github.com/facebookgo/clock v0.0.0-20150410010913-600d898af40a // indirect
-	github.com/facebookgo/ensure v0.0.0-20200202191622-63f1cf65ac4c // indirect
-	github.com/facebookgo/freeport v0.0.0-20150612182905-d4adf43b75b9 // indirect
-	github.com/facebookgo/stack v0.0.0-20160209184415-751773369052 // indirect
-	github.com/facebookgo/stats v0.0.0-20151006221625-1b76add642e4 // indirect
-	github.com/facebookgo/subset v0.0.0-20200203212716-c811ad88dec4 // indirect
-	github.com/go-git/go-git/v5 v5.4.2 // indirect
 	github.com/go-ldap/ldap v3.0.3+incompatible
 	github.com/go-redis/redis v6.15.9+incompatible
 	github.com/go-sql-driver/mysql v1.5.0
@@ -29,19 +21,21 @@ require (
 	github.com/lib/pq v1.9.0
 	github.com/magefile/mage v1.11.0 // indirect
 	github.com/mattn/go-sqlite3 v2.0.3+incompatible
-	github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e // indirect
 	github.com/schwarmco/go-cartesian-product v0.0.0-20180515110546-d5ee747a6dc9
 	github.com/sirupsen/logrus v1.8.0 // indirect
+	github.com/stretchr/testify v1.7.0 // indirect
 	github.com/syndtr/goleveldb v1.0.0
 	go.mongodb.org/mongo-driver v1.7.1
 	go.opencensus.io v0.23.0 // indirect
 	golang.org/x/crypto v0.0.0-20210421170649-83a5a9bb288b
 	golang.org/x/net v0.0.0-20210326060303-6b1517762897
 	golang.org/x/oauth2 v0.0.0-20210220000619-9bb904979d93
+	golang.org/x/sys v0.0.0-20210502180810-71e4cd670f79 // indirect
 	google.golang.org/api v0.40.0
 	google.golang.org/genproto v0.0.0-20210303154014-9728d6b83eeb // indirect
 	google.golang.org/grpc v1.36.0 // indirect
 	gopkg.in/asn1-ber.v1 v1.0.0-20181015200546-f715ec2f112d // indirect
+	gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c // indirect
 	gopkg.in/fsnotify.v1 v1.4.7
 	gopkg.in/mgo.v2 v2.0.0-20190816093944-a6b53ec6cb22
 	gopkg.in/yaml.v2 v2.4.0