Merge branch 'renovate/github.com-cenkalti-backoff-4.x' into 'master'

Update module cenkalti/backoff to v4

See merge request !13

go.mod

@@ -7,7 +7,7 @@ require (
 	git.autistici.org/id/usermetadb v0.0.0-20210112103616-f674f50555fc
 	github.com/boombuler/barcode v0.0.0-20170618053812-56ef0af91246 // indirect
 	github.com/bradfitz/gomemcache v0.0.0-20190913173617-a41fca850d0b
-	github.com/cenkalti/backoff v2.2.1+incompatible
+	github.com/cenkalti/backoff/v4 v4.1.0
 	github.com/coreos/go-systemd/v22 v22.1.0
 	github.com/go-ldap/ldap/v3 v3.2.4
 	github.com/go-sql-driver/mysql v1.4.0