update test_old_deps script

.buildkite/scripts/test_old_deps.sh

 #!/usr/bin/env bash
 
-# this script is run by buildkite in a plain `xenial` container; it installs the
+# this script is run by buildkite in a plain `bionic` container; it installs the
 # minimal requirements for tox and hands over to the py3-old tox environment.
 
 set -ex