go.mod: replace github.com/dgrijalva/jwt-go with .../golang-jwt/jwt

CVE-2020-26160 is a high-severity advisory reported against this module.

The dgrijalva package is no longer maintained but our legacy etcv2 backend
depends on it indirectly, via go.etcd.io/etcd/client. The golang-jwt
package is the blessed successor of the original, and has a v3 line which
is compatible with the v3 line of dgrijalva, and so through this replace
we can get a fix for the advisory without other significant behavior
change.

We've preserved the etcdv2 backend as-is on a best-effort basis in order
to support anyone who is already using it, but recommend that users switch
to etcdv3 or to some other backend for ongoing use.

We also have future plans to make state storage be a matter for provider
plugins rather than built in to Terraform CLI, at which point this backend
will either become obsolete or be factored out into its own plugin, at
which point we can remove this "replace" directive and the associated
dependency altogether.

go.mod

@@ -192,4 +192,10 @@ replace github.com/golang/mock v1.5.0 => github.com/golang/mock v1.4.4
 
 replace k8s.io/client-go => k8s.io/client-go v0.0.0-20190620085101-78d2af792bab
 
+// github.com/dgrijalva/jwt-go is no longer maintained but is an indirect
+// dependency of the old etcdv2 backend, and so we need to keep this working
+// until that backend is removed. github.com/golang-jwt/jwt/v3 is a drop-in
+// replacement that includes a fix for CVE-2020-26160.
+replace github.com/dgrijalva/jwt-go => github.com/golang-jwt/jwt v3.2.1+incompatible
+
 go 1.17

go.sum

@@ -172,9 +172,6 @@ github.com/davecgh/go-spew v0.0.0-20151105211317-5215b55f46b2/go.mod h1:J7Y8YcW2
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/dgrijalva/jwt-go v0.0.0-20160705203006-01aeca54ebda/go.mod h1:E3ru+11k8xSBh+hMPgOLZmtrrCbhqsmaPHjLKYnJCaQ=
-github.com/dgrijalva/jwt-go v3.2.0+incompatible h1:7qlOGliEKZXTDg6OTjfoBKDXWrumCAMpl/TFQ4/5kLM=
-github.com/dgrijalva/jwt-go v3.2.0+incompatible/go.mod h1:E3ru+11k8xSBh+hMPgOLZmtrrCbhqsmaPHjLKYnJCaQ=
 github.com/dimchansky/utfbom v1.1.0/go.mod h1:rO41eb7gLfo8SF1jd9F8HplJm1Fewwi4mQvIirEdv+8=
 github.com/dimchansky/utfbom v1.1.1 h1:vV6w1AhK4VMnhBno/TPVCoK9U/LP0PkLCS9tbxHdi/U=
 github.com/dimchansky/utfbom v1.1.1/go.mod h1:SxdoEBH5qIqFocHMyGOXVAybYJdr71b1Q/j0mACtrfE=
@@ -226,6 +223,8 @@ github.com/gogo/protobuf v1.1.1/go.mod h1:r8qH/GZQm5c6nD/R0oafs1akxWv10x8SbQlK7a
 github.com/gogo/protobuf v1.2.1/go.mod h1:hp+jE20tsWTFYpLwKvXlhS1hjn+gTNwPg2I6zVXpSg4=
 github.com/gogo/protobuf v1.2.2-0.20190723190241-65acae22fc9d h1:3PaI8p3seN09VjbTYC/QWlUZdZ1qS1zGjy7LH2Wt07I=
 github.com/gogo/protobuf v1.2.2-0.20190723190241-65acae22fc9d/go.mod h1:SlYgWuQ5SjCEi6WLHjHCa1yvBfUnHcTbrrZtXPKa29o=
+github.com/golang-jwt/jwt v3.2.1+incompatible h1:73Z+4BJcrTC+KczS6WvTPvRGOp1WmfEP4Q1lOd9Z/+c=
+github.com/golang-jwt/jwt v3.2.1+incompatible/go.mod h1:8pz2t5EyA70fFQQSrl6XZXzqecmYZeUEB8OUGHkxJ+I=
 github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b h1:VKtxabqXZkF25pY9ekfRL6a582T4P37/31XEstQ5p58=
 github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b/go.mod h1:SBH7ygxi8pfUlaOkMMuAQtPIUF8ecWP5IEl/CR7VP2Q=
 github.com/golang/groupcache v0.0.0-20160516000752-02826c3e7903/go.mod h1:cIg4eruTrX1D+g88fzRXU5OdNfaM+9IcxsU14FzY7Hc=