mirror-to-mirror.sh: replace mirror.f-droid.org with ftp.agdsn.de
mirror-to-mirror.sh: replace mirror.f-droid.org with ftp.agdsn.de

* ftp.agdsn.de is a new official mirror that receives push updates.
* mirror.f-droid.org is no more, it is now a DNS round robin redirector.

admin#247
admin#298

[skip ci]

f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver