Combine UI and dashboard PRs in GitHub Actions workflow

[superseb]

Currently we have rancher/ui and rancher/dashboard both use the GitHub Actions workflow Replace environment variable in file, which replaces the environment variable for UI and dashboard. The issue is that the one that is merged second, needs to be rebased because we are using branches in the repo itself and the commits are not in sync anymore. This is annoying and still uses too many resources.

We need to figure out how to combine those into one PR (this is based on an assumption we always have an UI and dashboard tag for a release (candidate))

[superseb]

This will be solved as soon as we have moved to Renovate, see an example here.