all: update go directive to 1.18

Done with:

go get [email protected]
go mod tidy
go fix ./...

Using go1.21.3.

With a manual change to keep golang.org/x/net/context testing itself,
not context in the standard library.

For golang/go#60268.

Change-Id: I00682bf7cf1e3ba4370e2a3e7f63dc245b294a36
Reviewed-on: https://go-review.googlesource.com/c/net/+/534241
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur

webdav/litmus_test_server.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build ignore
-// +build ignore
 
 /*
 This program is a server for the WebDAV 'litmus' compliance test at