Qt6 compatibillity

[ivaradi]

This patch adds compatibility with older Qt versions (from Qt 6.2.4)

[github-actions]

Artifact containing the AppImage: nextcloud-appimage-pr-7834.zip

SHA256 checksum: 90d2cea6c06a3fc1998553d3f628a6d74590eab572e37626ff9de8a6924ae33b

To test this change/fix you can download the above artifact file, unzip it, and run it.

Please make sure to quit your existing Nextcloud app and backup your data.

[sonarqubecloud]

Quality Gate failed

Failed conditions
3 Security Hotspots
72.9% Coverage on New Code (required ≥ 80%)
D Security Rating on New Code (required ≥ A)
D Maintainability Rating on New Code (required ≥ A)
222 New Code Smells (required ≤ 0)

See analysis details on SonarQube Cloud

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[Rello]

Hello István,

For transparency, I’d like to share the key points from our direct email exchange here:

We want to ensure that the desktop client can benefit from new Qt features across all platforms. Maintaining long-term compatibility with older Qt versions would limit our ability to do so.
We truly appreciate and value contributions from our community, including your work. However, we are unable to provide native package support for older Linux distributions. That’s why we distribute via AppImage and are also considering Snap and Flatpak packages.

Thank you for your effort on this PR! However, adding these compatibility checks introduces significant overhead and additional dependencies. In fact, we are currently working in the opposite direction—streamlining the codebase to enhance stability across platforms.

We truly appreciate your contributions and hope to collaborate on future improvements aligned with our development direction.

Best regards