THRIFT-5633: move xenial to old, deprecate xenial in travis build (#2690)

* move xenial to old, deprecate xenial in travis build

* add back stage of docker
diff --git a/lib/perl/build-cpan-dist.sh b/lib/perl/build-cpan-dist.sh
index ac0ff0b..b9250f4 100755
--- a/lib/perl/build-cpan-dist.sh
+++ b/lib/perl/build-cpan-dist.sh
@@ -2,7 +2,7 @@
 #
 # This script is intended to be used after tagging the repository and updating
 # the version files for a release.  It will create a CPAN archive.  Run this
-# from inside a docker image like ubuntu-xenial.
+# from inside a docker image like ubuntu-focal.
 #
 
 set -e