Refactor lib/private

[shdehnavi]

Summary

The required adjustments have been made to the following classes under /lib/private namespace:

• Setup.php
• Streamer.php
• StreamImage.php
• SubAdmin.php
• SystemConfig.php

The improvements:

• Using PHP8's constructor property promotion
• Adding return types
• Adding types to properties
• Updating doc blocks

Checklist

• Code is properly formatted
• Sign-off message is added to all commits

[skjnldsv]

Please resolve conflicts :)

[skjnldsv]

@shdehnavi thanks for the PR (and all the other ones too! 💪)
Unfortunately, we took a bit too much time to review and get it merged.
We rebased and merged as much as we can for the other, but this one was too conflicting.

If you used a tooling to create those, feel free to run it again now and ping me @skjnldsv straight away! I'll help you get it merged fast 🚀

Have a nice day :)