Initial configuration for branch metadata file

[richard-cox]

Summary

Fixes #15927

Occurred changes and/or fixed issues

• current plan is one file in master
• ties r/d branches to a rancher milestone
  • Next step is to create a workflow that will block PRs merging to the branch if they aren't the correct milestone
    • Prevent PRs from merging if they do not track the correct milestone for the target branch #16064
  • This has crossover with code freeze but also protects master from merges of PRs meant for all future releases (master not in code freeze and tracking 2.13.0... stops PRs targeting 2.14.0 from merging)
• includes other useful data for other use cases
  • e2e docker image
  • ui-dashboard-index/ui-index values to use for specific rancher branches

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
• The PR has a Milestone
• The PR template has been filled out
• The PR has been self reviewed
• The PR has a reviewer assigned
• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes
• The PR has been reviewed in terms of Accessibility
• The PR has considered, and if applicable tested with, the three Global Roles Admin, Standard User and User Base

[richard-cox]

@rak-phillip there's a few different aims of the file, but in terms of build and branching hopefully this can help? In an ideal world we just update this one file in master and everything else just works or automatically updates. Am sure there was a script that would update various parts on branching, is it checked in?

Update: We could add a code freeze flag to the branch and prevent merges as well, which would cover scenarios where the pr milestone is correct but no-one should merge