Specify the type for $heade

When we would like to extend our class for the EMailTemplate, we have a PHP error about the type of the `$header` variable.

Because, the `$header` property is typed : db3c731b0e/lib/private/Mail/EMailTemplate.php (L97) .

Signed-off-by: Baptiste Fotia <fotia.baptiste@hotmail.com>

admin_manual/configuration_server/email_configuration.rst

@@ -124,7 +124,7 @@ Let's assume that we need to override the email header::
 
    class MyClass extends EMailTemplate
    {
-      protected $header = <<<EOF
+      protected string $header = <<<EOF
          <table align="center" class="wrapper">
                // your theme email header modification
          </table>