Add Debian 13 (Trixie) support

[GreyXor]

Debian 13 (Trixie) is currently scheduled for release on August 9th, as per Debian's official announcement.

This issue is to track and prepare support for php Docker images based on Debian 13 once it becomes available in the official mirrors.

Keeping up with upstream base image releases ensures long-term supportability, access to the latest system libraries, and security improvements.

Thanks for your great work maintaining this image!

[tianon]

Similar to docker-library/python#1042 (comment), I'd be open to a draft PR, but not publishing/maintaining builds until GA.

[GreyXor]

Debian 13 Trixie is now GA
https://www.debian.org/News/2025/20250809