Require PHP 8.2 and support through PHP 8.5

[s3rgiosan]

Description of the Change

Raises the minimum PHP version to 8.2 and tests the plugin through PHP 8.5.

• Minimum PHP is 8.2 in composer.json (with config.platform.php set to 8.2), the plugin header, readme.txt, and the README.
• The PHPUnit workflow matrix covers PHP 8.2, 8.3, 8.4, and 8.5. WP_ENV_PHP_VERSION follows the matrix, so wp-env runs the suite on each version.
• A new PHP Static Analysis workflow runs PHPCompatibilityWP (testVersion 8.2-8.5) and PHPStan (level 2, phpVersion 8.2 to 8.5). New Composer scripts: lint, lint:compat, phpstan.
• Dev dependencies: 10up/phpcs-composer ^3.0.1, phpcompatibility/php-compatibility 10.0.0-alpha2, phpstan/phpstan ^2.1, szepeviktor/phpstan-wordpress ^2.0, phpunit/phpunit ^9.6. doctrine/instantiator is removed.
• shivammathur/setup-php is bumped to v2.37.2 in the PHPUnit and E2E workflows.
• Fixes found by PHPStan:
  • rest_prepare_{$post_type} is registered with add_filter, since prepare_links() returns the response.
  • The RelatedEntities docblocks type the relationship as PostToPost|PostToUser.
  • The @var annotations in DeletedItems name $p2u_table.

Closes #130

How to test the Change

1. composer install
2. composer lint:compat exits with no errors.
3. composer phpstan reports no errors.
4. WP_ENV_PHP_VERSION=8.5 npm run test:setup && npm run test:php passes. Repeat with 8.2.
5. The PHPUnit (8.2 to 8.5), E2E, and PHP Static Analysis workflows pass on this PR.

Follow-ups

• phpcompatibility/php-compatibility is pinned to 10.0.0-alpha2. Move to the stable 10.0.0 release once it ships.
• PHPStan runs at level 2. Raising the level is tracked separately.
• PHPUnit stays on 9.6. Upgrading to a newer major requires test and polyfill changes.

Changelog Entry

Added - PHPStan static analysis and a PHPCompatibility check for PHP 8.2 to 8.5, run in CI.
Changed - Bump minimum PHP version to 8.2 and test through PHP 8.5.
Changed - Update the PHPCS toolchain so the 10up ruleset and PHPCompatibilityWP sniffs install correctly.

Credits

Props @s3rgiosan

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.