Remove Brotli dependency

[edmorley]

Since the Brotli dependency adds ~7-8MB (depending on arch) to the app image (along with having to download/install the wheel during the build), and isn't currently used by the Getting Started guide, since it doesn't have any static assets that are compressible.

$ ls -alh /layers/heroku_python/venv/lib/python3.13/site-packages/
...
-rwxr-xr-x  1 heroku heroku 7.2M Jan  1  1980 _brotli.cpython-313-aarch64-linux-gnu.so

$ ls -al /workspace/staticfiles/ 
total 16
drwxr-xr-x 2 heroku heroku 4096 Jan  1  1980 .
drwxrwxrwx 5 heroku heroku 4096 Jan  1  1980 ..
-rw-r--r-- 1 heroku heroku 2217 Jan  1  1980 lang-logo.019c8743b7cf.png
-rw-r--r-- 1 heroku heroku  100 Jan  1  1980 staticfiles.json

See:
https://whitenoise.readthedocs.io/en/latest/django.html#brotli-compression

GUS-W-18705397.