🚚(backend) rename Impress to Meet

I have updated all references of "Impress" to "Meet".
Migrations were manually updated and not regenerated. Never-mind,
they all will be squashed before the first release.

I have also searched for reference to "Magnify", and replaced them
by "Meet".

While updating the backend sources, I have also fixed other parts of
the project, namely:
- Compose file
- Github documentation and CI
- Makefile commands

src/backend/core/api/serializers.py

@@ -1,4 +1,4 @@
-"""Client serializers for the impress core app."""
+"""Client serializers for the Meet core app."""
 from django.utils.translation import gettext_lazy as _
 
 from rest_framework import serializers