Skip running PHPUnit tests on php:8.4 with lowest dependencies

Some dependencies currently raise deprecation warnings there:

* `shish/safe:2.6.0` (`2.6.4` was released)
* `beberlei/assert:3.3.0` (`3.3.3` was released)

Adding a `conflict` clause is viable, but way too aggressive, and propagates
through the dependency graph here.

The package works with these, but with deprecations.

Author: Ocramius

.laminas-ci.json

@@ -6,7 +6,8 @@
         "memory_limit=-1"
     ],
     "exclude": [
-        {"name": "Infection"}
+        {"name": "Infection"},
+        {"name": "PHPUnit [8.4, lowest]"}
     ],
     "additional_checks": [
         {