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>

bcrypt/bcrypt.go

@@ -4,7 +4,7 @@
 
 // Package bcrypt implements Provos and Mazières's bcrypt adaptive hashing
 // algorithm. See http://www.usenix.org/event/usenix99/provos/provos.pdf
-package bcrypt // import "golang.org/x/crypto/bcrypt"
+package bcrypt
 
 // The code is a port of Provos and Mazières's C implementation.
 import (