Change flake8 pipeline's python image

Currently, the flake8 pipeline uses python3.8 image. This old image limits the supported versions of PySide6, so we need to update the image to a more recent one so that we can request more recent PySide6 versions.

.gitlab-ci.yml

@@ -23,7 +23,7 @@ before_script:
   - apt-get update && apt install -y libgl1-mesa-glx libxkbcommon-x11-0 libegl1 libdbus-1-3
 
 flake8:
-  image: python:3.8
+  image: python:3.11
   tags:
     - passoft
   stage: Build Env and Test