Simplify process for creating a new release branch

[turkenh]

Description of your changes

The existing process relying on rebase became more challenging with the recent state. With v1.14 release, I had to resolve conflicts for ~15 commits which even includes a revert.

This PR proposes a simplified one without any rebase, leveraging git diff.

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] ~Added backport release-x.y labels to auto-backport this PR, as appropriate.~

How has this code been tested

See https://github.com/upbound/crossplane/pull/109

[phisco]

would be interesting to see if using rerere would have simplified the rebasing: https://stackoverflow.com/questions/31401754/why-does-the-same-conflict-reappear-when-i-use-git-rebase

[turkenh]

It is strange. I rebased / force pushed and PR is closed saying it is merged and closed 🤔

[turkenh]

Opened https://github.com/upbound/universal-crossplane/pull/427