Removed the JupyterLab 3 environment from the Python 3.12 custom container; JupyterLab 4 is now the only JupyterLab environment and is always used.

Removed the JupyterLab 3 environment from the Python 3.12 custom container; JupyterLab 4 is now the only JupyterLab environment and is always used. The Python 3.10 images are unaffected.