[branch-51] Update version to 51.0.0, add Changelog (#18551)

## Which issue does this PR close?

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- part of #17558 

## Rationale for this change

Let's update the version numbers!

## What changes are included in this PR?

Update numbers and add changelog. 

I plan to merge this PR now and I will make a follow on PR (and forward
port it to main) with final touchups

## Are these changes tested?

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## Are there any user-facing changes?

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Author: alamb