Update module cenkalti/backoff to v4

go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/amoghe/go-crypt v0.0.0-20191109212615-b2ff80594b7f
 	github.com/bbrks/wrap/v2 v2.5.0
 	github.com/cenkalti/backoff v2.2.1+incompatible
+	github.com/cenkalti/backoff/v4 v4.1.0
 	github.com/coreos/go-systemd v0.0.0-20191104093116-d3cd4ed1dbcf
 	github.com/emersion/go-textwrapper v0.0.0-20200911093747-65d896831594
 	github.com/go-ldap/ldap/v3 v3.2.4
@@ -15,14 +16,17 @@ require (
 	github.com/gorilla/handlers v1.5.1
 	github.com/lunixbochs/struc v0.0.0-20200707160740-784aaebc1d40
 	github.com/miscreant/miscreant.go v0.0.0-20200214223636-26d376326b75
+	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/openzipkin/zipkin-go v0.2.5
+	github.com/pierrec/lz4 v2.0.5+incompatible // indirect
 	github.com/prometheus/client_golang v1.9.0
 	github.com/russross/blackfriday/v2 v2.1.0
-	github.com/shurcooL/sanitized_anchor_name v1.0.0 // indirect
 	github.com/theckman/go-flock v0.8.0
 	github.com/tstranex/u2f v1.0.0
 	go.opencensus.io v0.22.5
 	golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad
+	golang.org/x/oauth2 v0.0.0-20190226205417-e64efc72b421 // indirect
 	golang.org/x/sync v0.0.0-20201207232520-09787c993a3a
 	gopkg.in/asn1-ber.v1 v1.0.0-20181015200546-f715ec2f112d
+	gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15 // indirect
 )