remove ruby 4.0 tests, because it is blocked bei Redmine core

Author: alexandermeindl

.github/workflows/tests.yml

@@ -10,7 +10,7 @@ jobs:
 
     strategy:
       matrix:
-        ruby: ['3.2', '3.3', '3.4', '4.0']
+        ruby: ['3.2', '3.3', '3.4']
         redmine: ['6.1-stable', 'master']
         db: ['postgres', 'mysql']
         exclude: