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>

xts/xts.go

@@ -21,7 +21,7 @@
 //
 // Note that XTS is usually not appropriate for any use besides disk encryption.
 // Most users should use an AEAD mode like GCM (from crypto/cipher.NewGCM) instead.
-package xts // import "golang.org/x/crypto/xts"
+package xts
 
 import (
 	"crypto/cipher"