Branch master instead of using it as base

We recently ran into a lovely "feature" of github where this action created a pr from `master` to `develop`. The pr got out of date and there was a conflict of package-lock.json (due to dependabot prs going into develop first) and so I clicked the "update branch" button to have github recreate the pr for us, and what happened is github automatically merged `develop` into `master` in order to recreate the pr. The `master` branch had branch protections on that require a pr, but github ignored the protections and merged `develop` straight into `master` without a pr.

In order to mitigate this problem the action should create a brach from `master` and merge that into `develop`. We'll want to use a consistent naming scheme (e.g. `merge-master-develop` instead of `merge-master-develop-YYYY-MM-DD`) in order to detect if the branch has already been created.