Fix devcontainer, using correct Python version image

[msusag]

Description

This sets the Python image for devcontainer to a 3.8 version, to be aligned with current setup in cognite-sdk-python.

Reverts #1758

Also disables automated RenoveBot updated for the devcontainer image, to avoid dependency uppdates from RenovateBot to again use a "too new" version.

Checklist:

• Tests added/updated.
• Documentation updated. Documentation is generated from docstrings - these must be updated according to your change.
  If a new method has been added it should be referenced in cognite.rst in order to generate docs based on its docstring.
• Changelog updated in CHANGELOG.md.
• Version bumped. If triggering a new release is desired, bump the version number in _version.py and pyproject.toml per semantic versioning.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.90%. Comparing base (774e0d9) to head (f307687).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1803   +/-   ##
=======================================
  Coverage   92.90%   92.90%           
=======================================
  Files         121      121           
  Lines       17700    17700           
=======================================
+ Hits        16444    16445    +1     
+ Misses       1256     1255    -1     

see 3 files with indirect coverage changes