Add 'main' branch to cookiecutter`.github/workflows/main.yml` file

[fgomez828]

Description

Our cookiecutter templates make the .github/workflows/main.yml file available to us by default. This file triggers github actions to run tests for CI. However, on new projects, the master branch has been renamed main. We should include this branch in the list of branches for github to run tests when we push to it or open a PR against it.

Steps to reproduce

• In a new project created with one of our cookiecutter templates, make a simple change in a branch and commit the change.
• Verify that github actions are not triggered on opening up a PR against the main branch

[fgomez828]

It looks like this issue is only relevant for the Django cookiecutter template: https://github.com/datamade/how-to/blob/master/docker/templates/new-django-app/%7B%7Bcookiecutter.app_name%7D%7D/.github/workflows/main.yml