all: use the syntax //go:build instead of // +build

This commit removes the line syntax:

	// +build

by using just the syntax:

	//go:build

which is supported starting from Go 1.17 and has been considered
deprecated starting from Go 1.18.

Note that, in tests, the old syntax is kept. See #956.

cmd/scriggo/generate.go

@@ -202,7 +202,6 @@ func renderPackages(w io.Writer, dir string, sf *scriggofile, goos string, flags
 	// Skeleton for a package group.
 	const pkgsSkeleton = `// Code generated by scriggo command. DO NOT EDIT.
 //go:build {{.GOOS}} && {{.BaseVersion}} && !{{.NextGoVersion}}
-// +build {{.GOOS}},{{.BaseVersion}},!{{.NextGoVersion}}
 
 package {{.Name}}
 

playground/scriggo.wasm.go

@@ -1,5 +1,4 @@
 //go:build js && wasm
-// +build js,wasm
 
 package main
 

test/compare/cgo_off.go

@@ -1,5 +1,4 @@
 //go:build !cgo
-// +build !cgo
 
 package main
 

test/compare/cgo_on.go

@@ -1,5 +1,4 @@
 //go:build cgo
-// +build cgo
 
 package main
 

test/compare/run.go

@@ -414,8 +414,11 @@ func goldenCompare(testPath string, got []byte) error {
 	return nil
 }
 
-// isBuildConstraints reports whether line is a build contrain, as specified at
-// https://pkg.go.dev/go/build#hdr-Build_Constraints.
+// isBuildConstraints reports whether line is a build constraint, as specified
+// at https://pkg.go.dev/go/build#hdr-Build_Constraints.
+//
+// TODO(Gianluca): this function still needs to support the old format of build
+// tags. Please refer to the issue https://github.com/open2b/scriggo/issues/956.
 func isBuildConstraints(line string) bool {
 	line = strings.TrimSpace(line)
 	if !strings.HasPrefix(line, "//") {