add ubuntu 20.04 "focal" (LTS) to list of EOL distros

Ubuntu 20.04 reached end of support on April 30. There's still commercial
ESM (Extended Security Maintenance) support, but we don't account for that
in our packages; https://ubuntu.com/blog/ubuntu-20-04-eol-for-devicesional

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

install.sh

@@ -489,7 +489,7 @@ do_install() {
 		raspbian.buster|raspbian.stretch|raspbian.jessie)
 			deprecation_notice "$lsb_dist" "$dist_version"
 			;;
-		ubuntu.bionic|ubuntu.xenial|ubuntu.trusty)
+		ubuntu.focal|ubuntu.bionic|ubuntu.xenial|ubuntu.trusty)
 			deprecation_notice "$lsb_dist" "$dist_version"
 			;;
 		ubuntu.mantic|ubuntu.lunar|ubuntu.kinetic|ubuntu.impish|ubuntu.hirsute|ubuntu.groovy|ubuntu.eoan|ubuntu.disco|ubuntu.cosmic)