CI: Retire Ubuntu 20.04 builder

Ubuntu 20.04 has gone EOL as of April 2025, retire the CI builder.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #17403

.github/workflows/scripts/qemu-3-deps-vm.sh

@@ -171,9 +171,7 @@ case "$1" in
     echo "##[group]Install Ubuntu specific"
     sudo apt-get install -yq linux-tools-common libtirpc-dev \
       linux-modules-extra-$(uname -r)
-    if [ "$1" != "ubuntu20" ]; then
-      sudo apt-get install -yq dh-sequence-dkms
-    fi
+    sudo apt-get install -yq dh-sequence-dkms
     echo "##[endgroup]"
     echo "##[group]Delete Ubuntu OpenZFS modules"
     for i in $(find /lib/modules -name zfs -type d); do sudo rm -rvf $i; done