Fix set attributes for HTML node preview templates

Problem/Motivation

A follow up issue after
#3178907: Style the Node View mode preview bar
having

{% set ttributes = attributes.addClass(entity_type|clean_class ~ 'vartheme-node-preview-page') %}

Proposed resolution

Fix set attributes for HTML node preview templates.
Change to have following in the system html--node--preview.html.twig template.

{% extends "html.html.twig" %}
{% set attributes = attributes.addClass(entity_type|clean_class ~ 'vartheme-node-preview-page') %}

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ✅ Accessibility and Readability
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Update Release Notes and Update Helper on new feature change/addition
• ✅ Release varbase-9.0.12, vartheme_bs4-9.0.24

Varbase update type

• ✅ No Update
• ➖ Optional Update
• ➖ Forced Update
• ➖ Forced Update if Unchanged

User interface changes

Node preview in Ultrawide screens

Node preview in Large screens

Node preview in Small screens

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Issue #3346706: Fixed set attributes for HTML node preview templates