github.com/golang/protobuf/proto has been deprecated in favor of
google.golang.org/protobuf/proto, so migrate to the non-deprecated one.

Signed-off-by: Joe Richey <joerichey@google.com>

The version of 'staticcheck' (part of honnef.co/go/tools) we were
pinning is incompatible with Go 1.18, as per
https://github.com/dominikh/go-tools/issues/1165.

To allow 'make lint' to work with Go 1.18 and later, upgrade
honnef.co/go/tools to the latest version.

This requires that several other modules be upgraded too.

Bump up the required Go version to 1.16 so that we can assume that Go
modules are enabled by default.  Go 1.16 is the latest end-of-life
release, so this makes it so that we support the latest end-of-life
release (1.16), the current maintainance release (1.17), the current
release (1.18), and future releases.  This the same approach we took
when we last bumped up the required Go version.

Also update the ci.yml file to test with these versions.

The golang.org/x/crypto/ssh/terminal package is deprecated and merely a
wrapper around golang.org/x/term. Thus, use the latter directly.

Upgrade to get the new fscrypt declarations from Linux v5.4.

See: https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module

The tool code is never actually built, but the versions are still lock
in `go.mod` and `go.sum`. We can also simplify the Makefile.

As the Go community transitions to using the modules ecosystem,
we want to only support one way of managing dependencies.
So this change moves to only using Go modules for dependency management.

This means that our effective minimum Go version increases to Go 1.11.
To account for this, we also update:
  - the documentation
  - Makefile
  - CI scripts

Also add go version attrubute to go.mod