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ARROW-13343: [R] Update NEWS.md for 5.0

Closes #10723 from nealrichardson/news-5.0

Lead-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Co-authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>