message: ci: use Ubuntu 20.04 as the default Docker container
  All but 2 of the Ubuntu CIs are already using 20.04 or 21.04.
diff: @@ -47,7 +47,7 @@ export RUN_FUZZ_TESTS=${RUN_FUZZ_TESTS:-false}
  export EXPECTED_TESTS_DURATION_IN_SECONDS=${EXPECTED_TESTS_DURATION_IN_SECONDS:-1000}
  export CONTAINER_NAME=${CONTAINER_NAME:-ci_unnamed}
  -export DOCKER_NAME_TAG=${DOCKER_NAME_TAG:-ubuntu:18.04}
  +export DOCKER_NAME_TAG=${DOCKER_NAME_TAG:-ubuntu:20.04}
  # Randomize test order.
  # See https://www.boost.org/doc/libs/1_71_0/libs/test/doc/html/boost_test/utf_reference/rt_param_reference/random.html
  export BOOST_TEST_RANDOM=${BOOST_TEST_RANDOM:-1}
  