Replace deprecated ioutil with io and os

https://pkg.go.dev/io/ioutil is deprecated since Go 1.16.

tpl/openapi/openapi3/openapi3.go

@@ -16,7 +16,7 @@ package openapi3
 
 import (
 	"fmt"
-	"io/ioutil"
+	"io"
 
 	gyaml "github.com/ghodss/yaml"
 
@@ -74,7 +74,7 @@ func (ns *Namespace) Unmarshal(r resource.UnmarshableResource) (*OpenAPIDocument
 		}
 		defer reader.Close()
 
-		b, err := ioutil.ReadAll(reader)
+		b, err := io.ReadAll(reader)
 		if err != nil {
 			return nil, err
 		}