Modernize dev branch codebase to PHP 7.4+ minimum requirements

[zhyian]

Description

Mirrors the modernization done in version-3 (PR #142) for the dev branch. Raises the minimum PHP version to 7.4 and updates the codebase, tooling, and CI accordingly.

Codebase

• Typed properties, union types, and null-safe operators across all src/ and tests/ files
• Constructor promotion where applicable
• Replace @var docblocks with native type declarations

Static analysis

• Replace Psalm with PHPStan
• Resolve all PHPStan errors including PHP 7.4-specific edge cases

CI / workflows

• Split monolithic php-qa workflow into separate php-cs, php-lint, php-sa, and php-tests workflows
• Drop blocked Composer 2.7.x and 2.8.x from CI matrix

How has this been tested?

• All existing unit and integration tests pass
• PHPStan static analysis passes at configured level 10
• CI workflows validated against the matrix of supported PHP and Composer versions
• Tested with projects on my local setup

Types of changes

What types of changes does your code introduce?

Bug fixes

• Fix ENV_DIR config causing crash when not explicitly set: WpConfigStep, VcsIgnoreCheckStep, and EnvExampleStep used unwrap() which throws on the default null value. Changed to unwrapOrFallback('')(dbde344 and 5e40494).

New feature

• N/A

Breaking change (fix or feature that break existing functionality)

• Minimum PHP version raised from 7.1 to 7.4
• Minimum Composer version raised to 2.3+ (drops v1 support)
• Minimum PHPUnit version raised to 9.6+

Issues

Mirrors #142 for the dev branch.

Checklist:

• My code is tested
• My code follows the project code style
• My code has documentation (for new features or changed behavior)