[stable-25.0.x] ci: Bootstrap the test server on PHP 8.3 - #6564

Every Garm run against server master fails in "configure server":

    This version of Nextcloud requires at least PHP 8.3
    You are currently running 8.2.31. Please update your PHP version.

Nextcloud master raised its minimum to PHP 8.3, so the shallow-server image
pinned at php8.2 can no longer install it and the job dies before the emulator
is ever started.

Point the non-stable22 matrix entry at the php8.3 image. Note the tag differs:
the php8.2 image publishes ":1" while php8.3 currently only publishes ":latest",
so this is not a straight substitution of the version in the tag. That does mean
the pin is looser than before and should be tightened once a numbered tag is
published.

The stable22 branch of the expression keeps php8.0. It is unreachable today,
since the matrix only lists master, but it belongs with that server version if
the entry is ever restored.

Assisted-by: Claude Code:claude-opus-5
Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>