all: upgrade to google.golang.org/protobuf@v1.21.0 (#1081)

diff --git a/go.mod b/go.mod
index 2ce329e..f17daa8 100644
--- a/go.mod
+++ b/go.mod
@@ -4,5 +4,5 @@
 
 require (
 	github.com/google/go-cmp v0.4.0
-	google.golang.org/protobuf v1.20.2-0.20200320194150-9d397869d892
+	google.golang.org/protobuf v1.21.0
 )
diff --git a/go.sum b/go.sum
index b4226a0..2c4d6c7 100644
--- a/go.sum
+++ b/go.sum
@@ -12,5 +12,5 @@
 google.golang.org/protobuf v0.0.0-20200221191635-4d8936d0db64/go.mod h1:kwYJMbMJ01Woi6D6+Kah6886xMZcty6N08ah7+eCXa0=
 google.golang.org/protobuf v0.0.0-20200228230310-ab0ca4ff8a60/go.mod h1:cfTl7dwQJ+fmap5saPgwCLgHXTUD7jkjRqWcaiX5VyM=
 google.golang.org/protobuf v1.20.1-0.20200309200217-e05f789c0967/go.mod h1:A+miEFZTKqfCUM6K7xSMQL9OKL/b6hQv+e19PK+JZNE=
-google.golang.org/protobuf v1.20.2-0.20200320194150-9d397869d892 h1:LQPEPzH8usKk01hUU5qdXb17Tgjr/BLZhHy1h90Vd7U=
-google.golang.org/protobuf v1.20.2-0.20200320194150-9d397869d892/go.mod h1:47Nbq4nVaFHyn7ilMalzfO3qCViNmqZ2kzikPIcrTAo=
+google.golang.org/protobuf v1.21.0 h1:qdOKuR/EIArgaWNjetjgTzgVTAZ+S/WXVrq9HW9zimw=
+google.golang.org/protobuf v1.21.0/go.mod h1:47Nbq4nVaFHyn7ilMalzfO3qCViNmqZ2kzikPIcrTAo=