all: update go directive to 1.18

Done with:

go get go@1.18
go mod tidy
go fix ./...

Using go1.21.3.

While here, simplify package syncmap by taking advantage of knowing
that all supported Go versions will have the go1.9 build constraint
satisfied.

For golang/go#60268.

Change-Id: Ic0f24ab13ada6839573e55beee5516c1a6c7f3cc
Reviewed-on: https://go-review.googlesource.com/c/sync/+/534220
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

semaphore/semaphore_bench_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 semaphore_test
 
 import (