Fix bug on nbconvert: No module named 'notebook.services'

when running `jupyter nbconvert --execute --allow-errors --to pdf myfile.ipynb` an error occured: 
"""
  File "/home/user/.local/lib/python3.11/site-packages/jupyter_contrib_nbextensions/nbconvert_support/__init__.py", line 5, in <module>
    from .collapsible_headings import ExporterCollapsibleHeadings
  File "/home/user/.local/lib/python3.11/site-packages/jupyter_contrib_nbextensions/nbconvert_support/collapsible_headings.py", line 6, in <module>
    from notebook.services.config import ConfigManager
ModuleNotFoundError: No module named 'notebook.services'
"""
This change updates `collapsible_headings.py` to use the correct import `from jupyter_server.services.config import ConfigManager`

Author: jalsti

src/jupyter_contrib_nbextensions/nbconvert_support/collapsible_headings.py

@@ -3,7 +3,7 @@
 import json
 import os
 
-from notebook.services.config import ConfigManager
+from jupyter_server.services.config import ConfigManager
 
 from jupyter_contrib_nbextensions import __file__ as contrib_init