PHP 8 support #2388

Now that PHP 8 has been released we should update the app to work on php8.

Laravel framework v6.20 adds PHP8 support: https://github.com/laravel/framework/releases/tag/v6.20.0
This does bump the minimum version of php to 7.2.5: laravel/framework#34928

• Update framework.
• Go through dependencies and update where require.
• Update GitHub actions to cover php 8.
• Test locally

Due to php version change we'll need to add a notice to the update docs on release.