all: update tools that manage copyright headers

Update all code generation tools, and those that check for license
headers to use the new standard header.

Also update copyright statement in LICENSE file.

Fixes #6865

Signed-off-by: Will Norris <will@tailscale.com>

cmd/viewer/viewer.go

@@ -383,7 +383,7 @@ func main() {
 		genView(buf, it, typ, pkg.Types)
 	}
 	out := pkg.Name + "_view.go"
-	if err := codegen.WritePackageFile("tailscale/cmd/viewer", pkg, out, codegen.CopyrightYear("."), it, buf); err != nil {
+	if err := codegen.WritePackageFile("tailscale/cmd/viewer", pkg, out, it, buf); err != nil {
 		log.Fatal(err)
 	}
 	if runCloner {