Branch management

Juliaup follows a simple branching model: PRs with features and bug fixes are merged into the main branch, and new releases are tagged and released from the main branch.

We do follow a merge window approach: while a build is being tested on the release preview channel, we don't merge PRs into main.

TODO right now it is difficult to see whether this merge block is in place, one would essentially have to look at the workflow for tagged versions and see which environment gates have been approved. Ideally we would have an easier way to figure out whether the merge window is open or not...