Snapcraft: set version from new xml file

[LyzardKing]

Fix #7022
A change in jpackage has moved the version to an xml file.
We can parse that to read the version and set it in the snap package

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[LyzardKing]

@Siedlerchr The error in the CI is the upload part, the snap built succesfully (and set the version)

[Siedlerchr]

Yep I saw it. It's probably just a github secret thing which is not avaiable when non org members run the workflows. Nontheless, I added the switch to the image options in build.gradle to see if the cfg file then contains again the versinn number.

[LyzardKing]

As long as there's one file that we can parse it's fine.

[Siedlerchr]

My fix attempt didn't help, but yours work so I merge