chore: switch from iouitil to io.ReadAll (#272)

removed a TODO: switch to io.ReadAll and drop go1.15 support

example/client/service/service.go

@@ -5,7 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"html/template"
-	"io/ioutil"
+	"io"
 	"net/http"
 	"os"
 	"strings"
@@ -71,7 +71,7 @@ func main() {
 			}
 			defer file.Close()
 
-			key, err := ioutil.ReadAll(file)
+			key, err := io.ReadAll(file)
 			if err != nil {
 				http.Error(w, err.Error(), http.StatusInternalServerError)
 				return
@@ -161,7 +161,7 @@ func callExampleEndpoint(client *http.Client, testURL string) (interface{}, erro
 	}
 	defer resp.Body.Close()
 
-	body, err := ioutil.ReadAll(resp.Body)
+	body, err := io.ReadAll(resp.Body)
 	if err != nil {
 		return nil, err
 	}