all: bump go.mod version and drop compatibility shims

Also, remove the legacy import annotations.

Fixes golang/go#68147

Change-Id: Ibfcc9322f27224c0ba92ea42cd56912a7d8783fd
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/594256
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

ssh/testdata/doc.go

@@ -5,4 +5,4 @@
 // This package contains test data shared between the various subpackages of
 // the golang.org/x/crypto/ssh package. Under no circumstance should
 // this data be used for production code.
-package testdata // import "golang.org/x/crypto/ssh/testdata"
+package testdata