deepcopy: gen: match Go 1.17 build tag format

The Go team has started the process of replacing the old "// +build"
constraints with the new "//go:build" constraints. See the design
doc for more details here:

https://go.googlesource.com/proposal/+/master/design/draft-gobuild.md

What is relevant is that in Go 1.17, gofmt now synchronizes the two
styles. In particular, if gofmt is run against a deepcopy generated
file, it will add a new "//go:build" constraint.

See https://golang.org/doc/go1.17#gofmt for details.

This PR intends to make it so that the generated code is compliant with
the new gofmt style.

pkg/deepcopy/gen.go

@@ -175,7 +175,8 @@ type ObjectGenCtx struct {
 // writeHeader writes out the build tag, package declaration, and imports
 func writeHeader(pkg *loader.Package, out io.Writer, packageName string, imports *importsList, headerText string) {
 	// NB(directxman12): blank line after build tags to distinguish them from comments
-	_, err := fmt.Fprintf(out, `// +build !ignore_autogenerated
+	_, err := fmt.Fprintf(out, `//go:build !ignore_autogenerated
+// +build !ignore_autogenerated
 
 %[3]s