build: Check for NEWS and appdata updates for new releases

Bastien Nocera requested to merge wip/hadess/check-news into main

At least 4 different bugs were filed against the Flathub version of evince because Flatpak prominently shows the appdata version in the UI, making users think that there was a bug in the packaging, or that the package had not been updated.

Avoid this problem in the future by failing the build if the NEWS or appdata files aren't updated on release.

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