Make staging the default branch

[tweakch]

Description

As expected, secrets are not passed to the runner when the PR is coming from a fork.

The difference can be seen here:

• step 6 of the pre-merge check of Use environment variables for testing #5
• step 5 of a simple commit on master.

So we can only run the CI build when merging from local branches.

I am not a devops expert, but here are my suggestions on how to setup branches to streamline pull requests.

We could

• rename master to staging (keep as default)
• add a new branch main (not default)
• run the CI builds only on main
• tag releases on main #11

We must

• protect main and staging by pull request reviews and status checks
• run ci build only on main #10

This would make it possible to setup different workflows for PRs and release / integration tests

[mmahut]

The branches are renamed and protected.