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>

Author: dmitshur

errgroup/go120.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build go1.20
-// +build go1.20
 
 package errgroup
 

errgroup/go120_test.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build go1.20
-// +build go1.20
 
 package errgroup_test
 

errgroup/pre_go120.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build !go1.20
-// +build !go1.20
 
 package errgroup
 

go.mod

@@ -1,3 +1,3 @@
 module golang.org/x/sync
 
-go 1.17
+go 1.18

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 (

syncmap/go19.go

@@ -6,3 +6,13 @@
 // This was the prototype for sync.Map which was added to the standard library's
 // sync package in Go 1.9. https://golang.org/pkg/sync/#Map.
 package syncmap
+
+import "sync" // home to the standard library's sync.map implementation as of Go 1.9
+
+// Map is a concurrent map with amortized-constant-time loads, stores, and deletes.
+// It is safe for multiple goroutines to call a Map's methods concurrently.
+//
+// The zero Map is valid and empty.
+//
+// A Map must not be copied after first use.
+type Map = sync.Map