🐛 Fixed paste into product card descriptions not stripping header formatting

closes https://github.com/TryGhost/Ghost/issues/14018

- product card descriptions and toggle card content both use `<KoeingBasicHtmlTextarea>` which is a stripped down Koenig editor with basic formatting support where heading sections are not expected
- when pasting into or updating the content in `<KoeingBasicHtmlTextarea>`, convert any headings, blockquotes, or other markerable section types to standard paragraphs so the content formatting matches what is possible through the editing interface

ghost/admin/lib/koenig-editor/addon/components/koenig-basic-html-textarea.js

@@ -351,6 +351,7 @@ export default class KoenigBasicHtmlTextarea extends Component {
             // post.toRange() can fail if a list item was just removed
             // TODO: mobiledoc-kit bug?
         }
+
         markers.forEach((marker) => {
             let {markups} = marker;
             if (markups.length > 1 && marker.hasMarkup('code')) {
@@ -362,6 +363,11 @@ export default class KoenigBasicHtmlTextarea extends Component {
 
         // remove any non-markerable/non-list sections
         post.sections.forEach((section) => {
+            // headings are not supported so convert them to paragraphs
+            if (section.isMarkerable) {
+                section.tagName = 'p';
+            }
+
             if (!section.isMarkerable && !section.isListSection) {
                 let reposition = section === editor.activeSection;
                 postEditor.removeSection(section);