Change default PHP version to PHP 8.4 for Varbase

Problem/Motivation

Drupal Core supports PHP 8.4 starting from Drupal 10.4+ and Drupal 11.
Varbase already maintains active compatibility fixes for PHP 8.4 across its ecosystem, including updates for implicit nullable parameters and deprecated APIs.

Keeping the default PHP version below 8.4 creates an unnecessary gap between Varbase, Drupal Core, and modern hosting environments.

Proposed resolution

Update Varbase to use PHP 8.4 as the default PHP version, while continuing to support the currently supported PHP versions defined by Drupal Core.

This includes:

• Updating default PHP version references in documentation and tooling
• Aligning development, CI, and local environment defaults with PHP 8.4
• Ensuring consistency with ongoing PHP 8.4 compatibility fixes across Varbase components

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
• ➖ UX/UI designer responsibilities
• ➖ 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-10.1.0, varbase-10.0.9, varbase-9.1.13

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

Release notes snippet

• chore: #3563146 Change default PHP version to PHP 8.4 for Varbase