Update ubuntu container to 20.04

Author: softins

.github/workflows/autobuild.yml

@@ -205,22 +205,22 @@ jobs:
           - config_name:            Linux .deb amd64 (artifacts+codeQL)
             target_os:              linux
             building_on_os:         ubuntu-22.04
-            building_container:     ubuntu:18.04
+            building_container:     ubuntu:20.04
             base_command:           ./.github/autobuild/linux_deb.sh
             run_codeql:             true
             is_main_build_target:   true
 
           - config_name:            Linux .deb armhf (artifacts)
             target_os:              linux
             building_on_os:         ubuntu-22.04
-            building_container:     ubuntu:18.04
+            building_container:     ubuntu:20.04
             base_command:           TARGET_ARCH=armhf ./.github/autobuild/linux_deb.sh
             run_codeql:             false
 
           - config_name:            Linux .deb arm64 (artifacts)
             target_os:              linux
             building_on_os:         ubuntu-22.04
-            building_container:     ubuntu:18.04
+            building_container:     ubuntu:20.04
             base_command:           TARGET_ARCH=arm64 ./.github/autobuild/linux_deb.sh
             run_codeql:             false
 
@@ -293,7 +293,7 @@ jobs:
           xcode-version:            ${{ matrix.config.xcode_version }}
 
       - name:                       Set up base dependencies in container environment
-        if:                         matrix.config.building_container == 'ubuntu:18.04'
+        if:                         matrix.config.building_container == 'ubuntu:20.04'
         run:                        |
           set -eu
           apt-get -qq update