Stop pinning bullseye for PHP 8.1 & 8.2

[desrosj]

This removes the pinned bullseye version of Debian from the PHP 8.1 & 8.2 images.

The upstream base images have been updated and the underlying container is now using trixie, which matches PHP >= 8.3.

[desrosj]

• This seems to be causing failures within the E2E tests (see Test updated 8.1 & 8.2 containers wordpress-develop#10056).
• This fixes the AVIF issues seen after merging AVIF support improvements in GD #189 (reverted temporarily for PHP 8.1 & 8.2 in Revert AVIF changes for PHP 8.1 & 8.2 #197).
• This requires the ImageMagick tests failing after the Trixie update to be adjusted to @requires PHP < 8.1 (also see Test updated 8.1 & 8.2 containers wordpress-develop#10056).