Start a 9.1.x branch for Varbase Editor to support Bootstrap 5

Problem/Motivation

#3229295: Initialize Varbase Theme (Bootstrap 5 - SASS) theme ( Vartheme BS5 )
#3251531: Start a 9.1.x branch for Varbase and Varbase Project to support Bootstrap 5
#3251538: Start a 9.1.x branch for Varbase Media to support Bootstrap 5

Looking at the Migrating to v5. The Bootstrap 5 has many logical changes.
A new branch is logical. To have a space for development and space for update and migration path from Bootstrap 4 to Bootstrap 5

Drop of old styling classes in favour of new Bootstrap 5 CKEditor styling classes.

Proposed resolution

• Start a 9.1.x branch for Varbase Editor

Remaining tasks

• ✅ File an issue about this project
• ✅ Start a 9.1.x branch
• ✅ 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
• ➖ Update Release Notes and Update Helper on new feature change/addition
• ✅ Code review from 1 Varbase core team member
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Release varbase-10.0.0-alpha1, varbase_editor-10.0.0-alpha1

Varbase update type:

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

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A