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>

md4/md4.go

@@ -7,7 +7,7 @@
 // Deprecated: MD4 is cryptographically broken and should only be used
 // where compatibility with legacy systems, not security, is the goal. Instead,
 // use a secure hash like SHA-256 (from crypto/sha256).
-package md4 // import "golang.org/x/crypto/md4"
+package md4
 
 import (
 	"crypto"