Update matrix wp68

[aledesma-godaddy]

Description

• Update CI test matrices to reflect current supported versions
• Drop WordPress 6.5; add WordPress 6.8
• Add PHP 8.4 (and set as default where applicable)

Screenshots

N/A

Types of changes

• CI/configuration (non-breaking)

How has this been tested?

• Verified matrices and defaults in updated workflow files
• Ensured E2E matrix now targets WP 6.6, 6.7, 6.8 and PHP 7.4, 8.4
• Ensured PHP Unit matrix now targets WP 6.8 and PHP 7.4, 8.4
• Ensured default phpVersion in reusable workflows is 8.4
• Rely on GitHub Actions runs to validate across the new matrix

Acceptance criteria

• E2E workflow runs against WP 6.6, 6.7, 6.8 and PHP 7.4, 8.4 without CI errors
• PHP Unit workflow runs against WP 6.8 and PHP 7.4, 8.4 without CI errors
• No references to WP 6.5 remain
• Default PHP is 8.4 in reusable workflows

Checklist:

• My code is tested
• My code follows accessibility standards
• My code has proper inline documentation
• I've included any necessary tests
• I've included developer documentation
• I've added proper labels to this pull request