Update Release README.md with latest process (#18549)

## Which issue does this PR close?

- part of #17558



## Rationale for this change

As our releases have become more complex, it is important that our
release documentation reflects the current reality.
While working on the 51 release in
#17558 I found
it was somewhat out of date (didn't refer to the branches, for example)

## What changes are included in this PR?

Update readme
## Are these changes tested?

N/A
## Are there any user-facing changes?

No -- this is internal developer documentation

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Matt Butrovich <mbutrovich@users.noreply.github.com>

Author: 3 people