Prepare PHP 8.1-compatible Open Region fork

[Gruven]

Summary

• publish the fork as openregion/image while replacing intervention/image 4.x
• require PHP 8.1+ and select a compatible PHPUnit major for each PHP runtime
• depend on openregion/gif 5.x
• add PHP 8.1 to CI and use it as the Docker development baseline
• update package links, installation instructions, maintainership, and requirements
• remove PHP 8.3 typed class constants without changing their values

Verification

• Composer strict validation passed
• PHP 8.1 lint passed for 626 PHP files
• PHPStan and PHPCS passed on PHP 8.1
• local cross-package Composer consumer passed platform checks on PHP 8.1
• Docker build check and Compose configuration validation passed
• the full PHPUnit result is identical on PHP 8.1 and 8.2; the remaining 20 failures and one error are limited to JP2 support missing from the local ImageMagick build

Dependency order

Merge and release openregion/gif before consuming this branch as a released package.