Provide newer appstream in the PPA

[bbhtt]

Hello any chance, appstream>=0.16.3 can be provided in flatpak/stable or flatpak-unstable PPA for 22.04?

Ubuntu has it frozen to 0.15.2 on 22.04 but a newer appstream is required in some cases for flatpak-builder eg. flatpak/flatpak-builder#576 and flatpak-builder-lint.

[smcv]

Backporting appstream is not really in-scope for a PPA to get an up-to-date version of Flatpak, and could easily cause regressions in core Ubuntu packages.

If Flathub infrastructure needs a newer appstream, then I think it should either use Ubuntu 24.04 LTS (which has appstream 1.0.x), or add a separate PPA for that.