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

Problem/Motivation

#3229295: Initialize Varbase Theme (Bootstrap 5 - SASS) theme ( Vartheme BS5 ) →

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

Proposed resolution

• Start a 9.1.x branch for Varbase and a new branch for all changed modules and logic with Bootstrap 5
• Change the default theme from Vartheme BS4 to Vartheme BS5

Remaining tasks

• ✅ File an issue about this project
• ✅ Start a 9.1.x branch
  composer create-project vardot/varbase-project:9.1.x-dev PROJECT_DIR_NAME --stability dev --no-interaction
• ✅ 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 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