Release 3.0.15

I won't sign the tag for this release this time because of nixpkgs bugs:
  https://github.com/NixOS/nixpkgs/issues/115145
  https://github.com/NixOS/nixpkgs/issues/126848

I'll start signing tags again when a fix for this issue
(such as https://github.com/NixOS/nixpkgs/pull/283663 )
is in all stable nixpkgs channels.