refactor: move from io/ioutil to io and os packages

The io/ioutil package has been deprecated as of Go 1.16, see
https://golang.org/doc/go1.16#ioutil. This commit replaces the existing
io/ioutil functions with their new definitions in io and os packages.

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

alerting/provider/slack/slack.go

@@ -3,7 +3,7 @@ package slack
 import (
 	"bytes"
 	"fmt"
-	"io/ioutil"
+	"io"
 	"net/http"
 
 	"github.com/TwiN/gatus/v3/alerting/alert"
@@ -37,7 +37,7 @@ func (provider *AlertProvider) Send(endpoint *core.Endpoint, alert *alert.Alert,
 		return err
 	}
 	if response.StatusCode > 399 {
-		body, _ := ioutil.ReadAll(response.Body)
+		body, _ := io.ReadAll(response.Body)
 		return fmt.Errorf("call to provider alert returned status code %d: %s", response.StatusCode, string(body))
 	}
 	return err