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>

xtea/cipher.go

@@ -12,7 +12,7 @@
 // Deprecated: any new system should use AES (from crypto/aes, if necessary in
 // an AEAD mode like crypto/cipher.NewGCM) or XChaCha20-Poly1305 (from
 // golang.org/x/crypto/chacha20poly1305).
-package xtea // import "golang.org/x/crypto/xtea"
+package xtea
 
 // For details, see http://www.cix.co.uk/~klockstone/xtea.pdf