Update signal-desktop.deb to 7.12.0-beta.2

[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 124830

[flathubbot]

Build 124830 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/107796/org.signal.Signal.flatpakref

[github-actions[bot]]

🤖 This PR passed CI, and automerge-flathubbot-prs is true in flathub.json, so I'm merging it automatically. 🤖