Use godot-cpp 4.2 for the "Godot CPP" CI workflow

.github/workflows/godot_cpp_test.yml

@@ -7,7 +7,7 @@ env:
   # Used for the cache key. Add version suffix to force clean build.
   GODOT_BASE_BRANCH: master
   # Used for the godot-cpp checkout.
-  GODOT_CPP_BRANCH: '4.1'
+  GODOT_CPP_BRANCH: '4.2'
 
 concurrency:
   group: ci-${{github.actor}}-${{github.head_ref || github.run_number}}-${{github.ref}}-cpp-tests