Add prompt logic to handle if branch already exists

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Resolves:

• https://github.com/wordpress-mobile/release-toolkit-gutenberg-mobile/issues/209

When running the go run main.go release prepare all x.xx.x command and the branch already exists on the remote repo, the user is prompted if they want to continue. This PR adds the logic to delete the remote branch and continue cloning:

PR Changes

• Adds DeleteBranch GitHub interface
• When running prepare, moves git.Clone commands outside of the else statement to continue the automation (and for DRY-ness)

To test

1. Create a release, e.g. go run main.go release prepare all 1.108.0
2. Once the PR is created and the branch exists on the remote repo, run the same command again
3. At the prompt, choose either yes (continue) or no (exit)
4. If yes (continue), the remote branch should be deleted and the script resumes cloning the repo
5. If no (exit), the script should stop running and the remote branch remains

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@jhnstn Closing this one, we can re-open it or create a new issue if we need to.