Set composer conflict to avoid installing phpunit 11.5.29 and 11.5.30

Migrated issue

Reported by: fjgarlin

Related to !392 (merged)

Problem/Motivation

Once the upstream issue https://github.com/sebastianbergmann/phpunit/issues/6304 is fixed, we need to remove what was added here 0bb52ab6 and test that the phpunit jobs complete.

Related issue: Issue #3540591