PHPUnit 12 requires PHP 8.3+. Widen the constraint in the root and
each component composer.json so PHP 8.2 resolves to 11.5 (LTS) and
8.3+ to 12.x. Removes the per-job "Force PHPUnit 11.5" workaround
steps from CI.

| Q             | A
| ------------- | ---
| Branch?       | 4.3
| Tickets       | ∅
| License       | MIT
| Doc PR        | ∅