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>

cryptobyte/string.go

@@ -15,7 +15,7 @@
 //
 // See the documentation and examples for the Builder and String types to get
 // started.
-package cryptobyte // import "golang.org/x/crypto/cryptobyte"
+package cryptobyte
 
 // String represents a string of bytes. It provides methods for parsing
 // fixed-length and length-prefixed values from it.