After creating a new extension using the command ctb make ckanext extended ckanext-newextension and running make full-upgrade with the default Makefile that is generated, CKAN will not start due to the following error:
pkg_resources.VersionConflict: (redis 4.1.4 (/usr/lib/ckan/default/lib/python3.12/site-packages), Requirement.parse('redis<6.0.0,>=5.0.0'))
After I manually downgrade the Redis package by running pip install -U redis==5.3.1, CKAN starts correctly.