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

## Which issue does this PR close?

- part of apache#18072

## Rationale for this change

Get read for release

## What changes are included in this PR?

release version number and README

## Are these changes tested?
By CI

## Are there any user-facing changes?

New version

Author: alamb