fix: rename gunicornconf.py to gunicorn.conf.py to have it picked up automatically

Discussed and discovered in https://gitlab.developers.cam.ac.uk/uis/devops/digital-admissions/pools/smi/-/merge_requests/280#note_1584800

This MR fixes the naming of the gunicorn configuration. From the documentation:

The third source of configuration information is an optional configuration file gunicorn.conf.py searched in the current working directory or specified using a command line argument. Anything specified in this configuration file will override any framework specific settings.

Currently, the configuration in gunicornconf.py is completely ignored, and renaming it to gunicorn.conf.py will mean it will be picked up.