chore: Improve hot-fix & release process to generate a PR for merging back to main

[jimblanc]

Description of changes

This PR improves our hot-fix and release processes by reducing friction when unreleased changes are present on main.

It makes the following changes to the process:

• Generates a temporary merge branch to eliminate risk of potentially merging unreleased changes on main into release
• Creates a PR for merging the temporary merge branch back into main upon conclusion of a release

Benefits of this process:

• Don't need to worry about wiping out unreleased changes on main when executing a hot-fix
• No longer would need to freeze main when executing releases (beyond perhaps allowing e2e tests to stabilize)

This change will be accompanied with corresponding runbook changes & team training sessions.

Issue #, if available

Description of how you validated changes

Tested workflow on my fork.

Example of generated PR: jimblanc#5
Example workflow execution: https://github.com/jimblanc/amplify-js/actions/runs/8008326884/job/21874530318

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[HuiSF]

Why we couldn't create a merge PR directly from the release branch?

[jimblanc]

This is to reduce risk from the "Update branch" button, which in theory would pull in unreleased changes from main directly into the release branch. Even if we turn that branch protection rule off ("Require branch to be up to date before merging") on main, I still think it's a little safer.