Add `create_branch` and `checkout_and_pull` actions

[spencertransier]

What does it do?

Adds two new actions:

• create_branch
• checkout_and_pull

These are wrappers for the GitHelper methods of the same names. There are currently dozens of instances across the A8C apps where those methods are being called directly. As discussed in paaHJt-5v4-p2, we're moving away from calling Release Toolkit helper methods directly and instead work through actions.

I thought about adding unit tests for these actions or at least the underlying GitHelper methods, but the methods are basically running git commands and not much else. I'm open to discussing this, though! I also wasn't sure how we'd be able to test the pull part of the checkout_and_pull action in a test.

Checklist before requesting a review

• [X] Run bundle exec rubocop to test for code style violations and recommendations
• [X] Add Unit Tests (aka specs/*_spec.rb) if applicable
• [X] Run bundle exec rspec to run the whole test suite and ensure all your tests pass
• [X] Make sure you added an entry in the CHANGELOG.md file to describe your changes under the appropriate existing ### subsection of the existing ## Trunk section.
• [X] If applicable, add an entry in the MIGRATION.md file to describe how the changes will affect the migration from the previous major version and what the clients will need to change and consider.