This PR adds some improvements to the software updater:

1. Remove the OS selection combobox from the update dialog, it's not really serving a purpose. Users should just select the right installer on the website.
2. Only when the update is an official release, link to the openrocket.info website when you click "Install Update". For pre-releases, link to the GitHub release page. This is because normally, we do not upload pre-releases to the website.
3. When the update is a pre-release, add a warning text below the title, stating that it is a pre-releases and issues may be present.