all: add a go directive, set it to 1.18

Done with:

go get go@1.18 toolchain@none
go mod tidy
go fix ./...

Using go1.21.3.

Otherwise the module is considered to have an implicit go 1.16 line.
While here, drop the by-now-always-satisfied go1.7 build constraint.

For golang/go#60268.

Change-Id: Ie0216c6f22c5e25321042e1b7bdaa59f3c3ae843
Reviewed-on: https://go-review.googlesource.com/c/time/+/534243
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: dmitshur

go.mod

@@ -1 +1,3 @@
 module golang.org/x/time
+
+go 1.18

rate/rate_test.go

@@ -2,9 +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
-// +build go1.7
-
 package rate
 
 import (