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>

ocsp/ocsp.go

@@ -5,7 +5,7 @@
 // Package ocsp parses OCSP responses as specified in RFC 2560. OCSP responses
 // are signed messages attesting to the validity of a certificate for a small
 // period of time. This is used to manage revocation for X.509 certificates.
-package ocsp // import "golang.org/x/crypto/ocsp"
+package ocsp
 
 import (
 	"crypto"

ocsp/ocsp_test.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.7
-
 package ocsp
 
 import (