It's 2021 (#4906)

An update to the copyright years.

COPYBARA_INTEGRATE_REVIEW=https://github.com/overleaf/overleaf/pull/948 from lyh16:patch-1 409f925da3ce825ae108fdfca7ad1a10aa601119

Co-authored-by: lyh16 <41638369+lyh16@users.noreply.github.com>
GitOrigin-RevId: a004ee0db853c1189c3c5740d42a55107a2c88ed

README.md

@@ -74,4 +74,4 @@ Please see the [CONTRIBUTING](https://github.com/overleaf/overleaf/blob/master/C
 
 The code in this repository is released under the GNU AFFERO GENERAL PUBLIC LICENSE, version 3. A copy can be found in the `LICENSE` file.
 
-Copyright (c) Overleaf, 2014-2019.
+Copyright (c) Overleaf, 2014-2021.