Add three years of releases to evergreen-history

Add three years of releases to evergreen-history

evergreen-history.txt needs a little love. To start, I've added all releases
since 2019-04 to the file.

Outstanding to-dos:
* Add Release Manager election dates and names
* Add Oversight Board election dates and names - this could be retroactive
since we seemd to be inconsistent about adding those
* Add any new large sites that have adopted Evergreen with dates, e.g.
Westchester
* Should we add dates that TEP was incorporated and when we left the
Conservancy?

I can work on the above, but may not get to them all.

Signed-off-by: Kathy Lussier <kathy@kmlussier.com>