release-notes: add altcoins removal note

Release notes for #67687 (bc08b42d) [1]
Related issue: #25025 [2]

[1] https://github.com/NixOS/nixpkgs/issues/67687
[2] https://github.com/NixOS/nixpkgs/issues/25025



Suggested-by: @mmahut
Signed-off-by: William Casarin <jb55@jb55.com>