Add Bunny CDN API Key to encrypted group_vars/all.yml

I'm working on a PR that runs a curl request to Bunny's API to bust the cache after we rsync www.php.net to the server. I'm unable to create Bunny API keys:

<img width="1022" height="235" alt="Image" src="https://github.com/user-attachments/assets/57ce9a1d-d8b0-44a3-951d-cd2f067126f8" />

Please add `bunny_api_key` to `/inventory/php/group_vars/all.yml`, thanks!