all: update go directive to 1.18

Done with:

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

Using go1.21.3.

Also update code generators to use only the new go:build lines,
not the old +build ones.

For golang/go#60268.

Change-Id: I832981d939ab0aad5c941555a6a4cd005958a189
Reviewed-on: https://go-review.googlesource.com/c/text/+/534242
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur

cases/gen.go

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 //go:build ignore
-// +build ignore
 
 // This program generates the trie for casing operations. The Unicode casing
 // algorithm requires the lookup of various properties and mappings for each

cases/gen_trieval.go

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

cases/icu.go

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

cases/icu_test.go

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