Update code-server and switch channel for OpenCV

• Update code-server to 4.95.2
• Switch the channel and package for OpenCV from fastai::opencv-python-headless to conda-forge::py-opencv=*=headless*, since FastAI channel is rather slow and does not provide a package for Python 3.12 at the time of writing.
• Remove an obsolete pip update for Keras, which was useful only at the transition from Keras 2 to Keras 3