Bump go.mod and docker go version to 1.16 (#18280)

* Bump go.mod go version to 1.16

* Bump docker build go version to 1.16.7

build/Dockerfile.buildenv

@@ -1,3 +1,3 @@
-FROM golang:1.15.5
+FROM golang:1.16.7
 
 RUN apt-get update && apt-get install -y make git apt-transport-https ca-certificates curl software-properties-common build-essential zip xmlsec1 jq

go.mod

@@ -1,6 +1,6 @@
 module github.com/mattermost/mattermost-server/v6
 
-go 1.15
+go 1.16
 
 require (
 	code.sajari.com/docconv v1.1.1-0.20210427001343-7b3472bc323a