[FIX] web_editor: restore mass_mailing's Mail Debug tab's embedded style

An error during the conversion of the html field to Owl caused the
embedded style of the Mail Debug tab to be lost because they were inside
a `<link>` element rather than in a `<style>` element.

closes odoo/odoo#115863

X-original-commit: 218909a339b2f3937e6d6f72fd0c6ff62aa305ce
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/web_editor/static/src/js/backend/html_field.js

@@ -463,7 +463,7 @@ export class HtmlField extends Component {
                     cwindow.document.head.append(link);
                 }
                 for (const cssContent of asset.cssContents) {
-                    const style = cwindow.document.createElement('link');
+                    const style = cwindow.document.createElement('style');
                     style.setAttribute('type', 'text/css');
                     const textNode = cwindow.document.createTextNode(cssContent);
                     style.append(textNode);