CI/CD: Validate MetaInfo files

[DavidoTek]

Is your feature request related to a problem? Please describe.
When a new version is released, the MetaInfo file gets updated (e.g., #427).
If this file contains an error, the Flathub build will fail.

Describe the solution you'd like
Automatically validate the MetaInfo file using CI/CD using appstreamcli validate
See https://docs.flatpak.org/en/latest/conventions.html#metainfo-files

Describe alternatives you've considered
None, don't validate and hope nothing breaks.

Additional context
Should the validation be run on every commit? Can we check if the MetaInfo file was modified and only then run the validation?