refactor: update code base to php 8 and set phpstan lvl to 6

.gitlab-ci.yml

.prettierrc.json

Dockerfile

INSTALL.md

app/Authorization/FlatAuthorization.php

app/Config/ActivityPub.php

app/Config/Analytics.php

app/Config/Database.php

app/Config/Exceptions.php

app/Config/Filters.php

app/Config/Kint.php

app/Config/Logger.php

app/Config/Routes.php

app/Config/Services.php

app/Config/View.php

app/Controllers/ActorController.php

app/Controllers/Admin/BaseController.php

app/Controllers/Admin/ContributorController.php

app/Controllers/Admin/EpisodeController.php

app/Controllers/Admin/EpisodePersonController.php