Fix CI build failure by updating to go 1.16

[bhamail]

Overnight CI build started to fail like so:

go mod tidy
github.com/sonatype-nexus-community/ahab/cmd imports
	github.com/spf13/viper imports
	github.com/spf13/afero imports
	io/fs: package io/fs is not in GOROOT (/usr/local/go/src/io/fs)
github.com/sonatype-nexus-community/ahab/cmd imports
	github.com/spf13/viper imports
	github.com/spf13/afero tested by
	github.com/spf13/afero.test imports
	testing/fstest: package testing/fstest is not in GOROOT (/usr/local/go/src/testing/fstest)
make: *** [Makefile:33: deps] Error 1

This appears to be related to this issue: golang/go#40067

The issue is solved by updating to use golang 1.16.

I also remove some obsolete replace directives that are fixed by upgrading.

cc @bhamail / @DarthHater