Downgrade tailwind to 3.6

(this is the last version that supports Python 3.9, which we want to
support as it's not EOL for a full more year)
https://github.com/timonweb/django-tailwind/issues/210

Afterwards, I went through a full cycle of:

```
rm theme/static_src/package-lock.json theme/static_src/node_modules/ -rf
python manage.py tailwind install
python manage.py tailwind build
```

but this did not change any of our generated stuff.

requirements.txt

@@ -1,7 +1,7 @@
 gunicorn
 Django==4.2.*
 sentry-sdk==2.15.*
-django-tailwind==3.8.*
+django-tailwind==3.6.*
 jsonschema==4.19.*
 semver==3.0.*
 django-admin-autocomplete-filter==0.7.*