Standardize on zstd compression

[probonopd]

With recent initiatives in the AppImage ecosystem to simplify things and increase compatibility, we are considering to standardize on zstd compression for squashfs-based ("type 2") AppImages.

Would it be possible for appimage-builder to use zstd exclusively, or at least by default?

Reference:

• https://github.com/AppImage/appimage.github.io/pull/3200

[azubieta]

Yes it's possible. Will do the change.

[probonopd]

Thank you very much @azubieta. Greetings!