Upgrade Castopod to PHP 8.1

Is your feature request related to a problem? Please describe

PHP 8.0's end of life (EOL) will be at the end of 2023 (security fixes included). Given the v1 Castopod will be released around the end of 2022, this will leave 1 year before needing to migrate to 8.1

This would introduce a breaking change quite soon after users coming to v1.

• Plus, 8.1 comes with the enums feature, this could help ensure a better code base.

Describe the solution you'd like

Migrate to 8.1 (EOL at the end of 2024) and inform users to update their php version if they are on 8.0

Describe alternatives you've considered

Update all enumeration fields to use the new enum method. (eg. Podcast status, episode status, ...)

Additional context

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