Support using a different default branch name instead of `master`

[wwahammy]

In general, master is a pretty problematic branch name and, by and large, SfG has moved to using main as our default branch. Unfortunately, Pantheon doesn't allow default branch names other than master. To work around this limitation, Deploy to Pantheon should support treating a branch other than master as the default on Github but deploy that branch to Pantheon as master.

For this, we should

• Add a workflow input for defining what the default branch is
  • Open question: what should the default branch on the repo be if none provided?
  • Open question: could we get the default branch from github itself if none provided? (likely a second issue)
• Make sure Deploy to Pantheon knows which branch is default so it knows to push to master on Pantheon