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>

hkdf/hkdf.go

@@ -8,7 +8,7 @@
 // HKDF is a cryptographic key derivation function (KDF) with the goal of
 // expanding limited input keying material into one or more cryptographically
 // strong secret keys.
-package hkdf // import "golang.org/x/crypto/hkdf"
+package hkdf
 
 import (
 	"crypto/hmac"