Base: PHP 8.x upgrade

[AdSchellevis]

Important notices

Before you add a new report, we ask you kindly to acknowledge the following:

• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

Is your feature request related to a problem? Please describe.

With PHP 7.4 going eol at the end of this year, we should start migrating to the new 8.x version.

Additional context

• https://www.php.net/supported-versions.php
• https://www.php.net/manual/en/migration80.php