Change log for 21.1.0-beta1.
Change log for 21.1.0-beta1.

- Fix release dates for v20 releases.

r00t-user/SeriesGuide,UweTrottmann/SeriesGuide,0359xiaodong/SeriesGuide,epiphany27/SeriesGuide,UweTrottmann/SeriesGuide,hoanganhx86/SeriesGuide,artemnikitin/SeriesGuide