Branch master instead of using it as base

[straker]

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 The pr got out of date and there was a conflict of package-lock.json (due to dependabot prs going into develop first) and so I clicked the "update branch" button to have github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.