🩹 fix: Clean output buffering up to initial level

[tombroucke]

This PR should resolve #406

The registerWordPressRoute method deletes all levels of output buffering. Caching plugins try to buffer the whole output and write that to a file. Output buffering is started by caching plugins in {wp-content|app}/advanced-cache.php, and saved in a callback for that ob_start(). As acorn is deleting all buffers from all levels in Bootable::registerWordPressRoute(), the output is empty by the time the caching plugin's callback is called.

With this PR, we get the initial ob level, and delete buffers up until this level.

I have tested this fix with

• WP Rocket
• Super Page Cache
• https://roots.io/acorn/docs/routing/#basic-routing-example
• wordpress routing + livewire
  and didn't encounter any issues, but please test thoroughly, as this is quite a critical change.

[tombroucke]

This also fixes #423

[QWp6t]

I like it! Thanks! 🙏

[RafaelKr]

Very clean, thank you very much!

[dsturm]

FYI: I ran into an issue with RealCookieBanner, where using Wordpress routing would fail to load an essential taxonomy and therefor generated multiple PHP errors. This PR fixed this problem.