github: hello_world_multiplatform: switch to python 3.12

Switch the workflow to setup python 3.12, not sure why this was left
out.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

.github/workflows/hello_world_multiplatform.yaml

@@ -56,7 +56,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5.6.0
         with:
-          python-version: 3.11
+          python-version: 3.12
 
       - name: Setup Zephyr project
         uses: zephyrproject-rtos/action-zephyr-setup@b2453c72966ee67b1433be22b250348d48283286 # v1.0.7