[mod] Drop Ubuntu 16.04 (Xenial Xerus) support

EOL of Ubuntu 16.04 (Xenial Xerus) is in April 2021 but we do not support 16.04
since a longer time.  Issues are comming from dependencies (e.g. pip)::

    [searx-ubu1604]   |searx| SyntaxError: invalid syntax
    [searx-ubu1604]   |searx| Traceback (most recent call last):
    [searx-ubu1604]   |searx|   File "/usr/local/searx/searx-pyenv/bin/pip", line 7, in <module>
    [searx-ubu1604]   |searx|     from pip._internal.cli.main import main
    [searx-ubu1604]   |searx|   File "/usr/local/searx/searx-pyenv/lib/python3.5/site-packages/pip/_internal/cli/main.py", line 60
    [searx-ubu1604]   |searx|     sys.stderr.write(f"ERROR: {exc}")
    [searx-ubu1604]   |searx|                                    ^
    [searx-ubu1604]   |searx| SyntaxError: invalid syntax

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>