fix: changed alignment from middle to center

[dotmarn]

This PR addresses an issue with the latest release, v6.5.0, where Avatar::create('John Doe')->toBase64(); throws an invalid value for alignment error.

Result

[adeiming]

An upvote for this PR. This completely fixes the InvalidArgumentException: Invalid value for alignment issue when running laravolt/avatar alongside Intervention Image v4.

The hardcoded 'middle' string is not present in Intervention v4's native Alignment Enum, throwing a native ValueError. Changing it to separate align('center') and valign('center') methods resolves the issue perfectly. Looking forward to seeing this merged!