qt5-qtwebengine: Update qtwebengine.git to v5.15.18-lts

[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 157792

[flathubbot]

Build 157792 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/140882/io.qt.qtwebengine.BaseApp.flatpakref