Change downloads to NewPipe repo and remove NewPipe Legacy

[TobiGr]

This PR

• removes the information about NewPipe Legacy which is soon going to be removed from the NewPipe website. (DEPLOY WITH THE WEBSITE UPDATE TeamNewPipe/website#299)
• adds information about hash sums, requires https://gitlab.com/fdroid/fdroid-website/-/merge_requests/841 to be merged
• uses NewPipe's F-Droid repo for APK downloads instead of GitHub's download server, replaces GitHub releases with NewPipe's F-Droid repository completely, because GitHub does not provide hash sums directly (we could upload them to GH releases as sparate file anyway)

ToDo:

• [x] remove all code related to NP Legacy