release 2021-04-01

Set "compatibility version" and "current version".

For #304.

Change-Id: I74e2720ecad9a0b22e8cd73dbffe2e02599827c5
Reviewed-on: https://code-review.googlesource.com/c/re2/+/58450
Reviewed-by: Paul Wankadia <junyer@google.com>