Update LICENSE text, year, contributors, and authors list

This updates the template in a few ways:

- Updates the year to 2021
- Removes the text "All rights reserved." and removes the 3rd clause
of the BSD license in response to guidance
- Updates default AUTHORS entry from Google Inc to Google LLC
- Updates CONTRIBUTING to current text

Change-Id: I216f25a81d079426bc7b751560d243a3d2a2080a
Reviewed-on: https://turquoise-internal-review.googlesource.com/c/staging/oss_template/+/301530
Reviewed-by: Adam Barth <abarth@google.com>