If a package is "kept back", that means the
upgrade command will not upgrade it because doing so would require installing or removing other packages. You can override this with the
sudo apt-get install fwupd-signed
It will list which packages it has to install or remove in order to upgrade that, so make sure it's not removing anything you care about.
In your case, it looks like you have versions of fwupd and fwpupd-signed that were provided by Pop_OS!, but you no longer have the Pop_OS! repositories configured. If you are using Pop, you should add their repositories, then try doing
sudo apt-get update && sudo apt-get upgrade again: https://apt.pop-os.org/
If you're not using Pop, then you can force Ubuntu to install its versions instead. Try:
sudo apt install fwupd=1.4.7-0~20.10.2 fwupd-signed=1.30.2+1.4.7-0~20.10.2