remove Xenial support

[dirk-thomas]

Fixing an issue with Focal / Python 3.8:

Traceback (most recent call last):
  File "run_ros2_batch.py", line 32, in <module>
    sys.exit(main())
  File "/home/jenkins-agent/workspace/ci_linux/ros2_batch_job/__main__.py", line 153, in main
    if sys.platform.lower().startswith('linux') and platform.linux_distribution()[2] == 'xenial':
AttributeError: module 'platform' has no attribute 'linux_distribution'

Before:
After:

[nuclearsandwich]

I'm mildly curious whether the undefined attribute is an artifact of Focal's pre-release status or if it's something we can expect in the released version as well. But this solves the issue and no currently supported ROS 2 distribution supports Xenial.

[dirk-thomas]

I'm mildly curious whether the undefined attribute is an artifact of Focal's pre-release status or if it's something we can expect in the released version as well.

The function has simply been removed in Python 3.8 after being deprecated since Python 3.5: https://docs.python.org/2/library/platform.html#platform.linux_distribution