tooling: use go version 1.16 as minimum version (#6642)

test/docker/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.15
+FROM golang:1.16
 
 # Grab deps (jq, hexdump, xxd, killall)
 RUN apt-get update && \

test/e2e/docker/Dockerfile

@@ -1,7 +1,7 @@
 # We need to build in a Linux environment to support C libraries, e.g. RocksDB.
 # We use Debian instead of Alpine, so that we can use binary database packages
 # instead of spending time compiling them.
-FROM golang:1.15
+FROM golang:1.16
 
 RUN apt-get -qq update -y && apt-get -qq upgrade -y >/dev/null
 RUN apt-get -qq install -y libleveldb-dev librocksdb-dev >/dev/null