Remove Ubuntu 20 support

Ubuntu 20 is EOL [1] unless people pay for support. Therefore, remove it
from build_helper. People can still use it, but might need to install a
more recent cmake version (which the next commit will upgrade). The rest
should be the same.

[1] https://ubuntu.com/about/release-cycle

cmake_targets/tools/build_helper

@@ -105,7 +105,6 @@ check_supported_distribution() {
     case "$distribution" in
         "ubuntu24.04") return 0 ;;
         "ubuntu22.04") return 0 ;;
-        "ubuntu20.04") return 0 ;;
         "debian11")    return 0 ;;
         "debian12")    return 0 ;;
         "fedora41")    return 0 ;;
@@ -281,7 +280,7 @@ check_install_usrp_uhd_driver(){
         $SUDO apt-get update
         $SUDO apt-get -y install python3-tk  $boost_libs_ubuntu libusb-1.0-0-dev
         case "$(get_distribution_release)" in
-          "ubuntu20.04" | "ubuntu22.04")
+          "ubuntu22.04")
             $SUDO apt-get -y install libuhd-dev libuhd4.5.0 uhd-host
             ;;
         esac
@@ -441,7 +440,7 @@ check_install_additional_tools (){
   local optional_packages=""
   if [[ "$OS_BASEDISTRO" == "debian" ]]; then
     case "$(get_distribution_release)" in
-        "ubuntu20.04" | "ubuntu22.04" | "debian11" | "debian12" )
+        "ubuntu22.04" | "debian11" | "debian12" )
             optional_packages="python3 python3-pip python3-dev python3-scipy python3-matplotlib python3-pyroute2 universal-ctags"
             ;;
     esac