gitlab-ci: modernize ubuntu_lts_ppa job using updates to PPA
gitlab-ci: modernize ubuntu_lts_ppa job using updates to PPA

The PPA now provides apksigner that does not need binfmt, and defusedxml is
a hard Depends:.

f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server