fix: restore PHP dependency constraints to match lockfile

[bfiessinger]

Motivation

• CI composer install on PHP 7.4 failed because recent require-dev bumps upgraded PHPUnit and related packages to releases that require PHP >=8.2, causing a lockfile/platform mismatch.
• Re-aligning composer.json with the committed composer.lock restores a reproducible install for the repository's CI matrix without regenerating lockfiles behind the restricted network.

Description

• Reverted root composer.json require-dev constraints to the lockfile-aligned versions: mythemeshop/wordpress-helpers -> ^1.1, phpunit/phpunit -> ^11.3, yoast/phpunit-polyfills -> ^3.0, squizlabs/php_codesniffer -> ^3.10, and phpcompatibility/phpcompatibility-wp -> ^2.1.
• Restored php-scoper/composer.json to require humbug/php-scoper ^0.15.0 to match the repository’s existing setup.
• Committed the changes and created the updated PR describing the lockfile-alignment fix.

Testing

• Ran composer validate --no-check-all --no-check-publish, which completed successfully.
• Ran composer install --dry-run --no-interaction, which completed successfully and reported installs consistent with the checked-in composer.lock.
• An attempted composer update to refresh lockfiles failed due to outbound proxy/registry connection errors, so lockfiles were intentionally left unchanged.

---

Codex Task