Disable StartupNotify to fix alt-tabbing in GNOME

Having [`StartupNotify` set to true in the `.desktop` file](https://github.com/flathub/com.visualstudio.code/blob/master/com.visualstudio.code.desktop#L8) is causing [problems with mutter](https://gitlab.gnome.org/GNOME/mutter/-/issues/2689) where you can't alt-tab to the application until it has timed out.

Would it be possible to set it to `false`? [Upstream Code seems to do the same.](https://github.com/microsoft/vscode/blob/main/resources/linux/code.desktop#L8)

There is also [a secondary desktop file](https://github.com/flathub/com.visualstudio.code/blob/master/com.visualstudio.code-url-handler.desktop) which might need the same treatment.

Thanks!