signal-desktop: Update signal-desktop.deb to 7.15.0-beta.1

[github-actions[bot]]

🤖 This pull request was automatically generated by flathub-infra/flatpak-external-data-checker. Please open an issue if you have any questions or complaints. 🤖

[flathubbot]

Started test build 130874

[flathubbot]

Build 130874 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/113869/org.signal.Signal.flatpakref