[branch-50] Prepare 50.3.0 release version number and README (#18173) (#18182)

## Which issue does this PR close?

- Related to #18072

## Rationale for this change
Bring 50.3.0 changes to main

## What changes are included in this PR?

- cherry-pick #18173 into main:
- CHANGELOG
- Version number change

## Are these changes tested?
Yes by CI

## Are there any user-facing changes?

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Author: alamb