Update test matrix to use only prefer-stable stability

Testbench/PHPUnit causing error in older versions

Author: hapidjus

.github/workflows/run-tests.yml

@@ -23,7 +23,7 @@ jobs:
         os: [ubuntu-latest, windows-latest]
         php: [8.4, 8.3]
         laravel: [12.*, 11.*]
-        stability: [prefer-lowest, prefer-stable]
+        stability: [prefer-stable]
         include:
           - laravel: 12.*
             testbench: 10.*