oss: Update README copyright year to 2021
Problem
We've crossed into a new year and our copyright ranges are now out of date.
Solution
Update them to 2021.
JIRA Issues: CSL-10577
Differential Revision: https://phabricator.twitter.biz/D606603
6c8e961 develop 30d2d47
6c8e961
develop
30d2d47
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