Remove support to bionic 18.04

[guimaluf]

The current charm page says hardware-observer is supported in Bionic 18.04, but it is not due the lack of python3.8 support for ops python library.

unit-hardware-observer-31: 12:33:25 WARNING unit.hardware-observer/31.install Traceback (most recent call last):
unit-hardware-observer-31: 12:33:25 WARNING unit.hardware-observer/31.install   File "./src/charm.py", line 10, in <module>
unit-hardware-observer-31: 12:33:25 WARNING unit.hardware-observer/31.install     import ops
unit-hardware-observer-31: 12:33:25 WARNING unit.hardware-observer/31.install   File "/var/lib/juju/agents/unit-hardware-observer-31/charm/venv/ops/__init__.py", line 171, in <module>
unit-hardware-observer-31: 12:33:25 WARNING unit.hardware-observer/31.install     from . import pebble  # type: ignore # noqa: F401
unit-hardware-observer-31: 12:33:25 WARNING unit.hardware-observer/31.install   File "/var/lib/juju/agents/unit-hardware-observer-31/charm/venv/ops/pebble.py", line 44, in <module>
unit-hardware-observer-31: 12:33:25 WARNING unit.hardware-observer/31.install     from typing import (
unit-hardware-observer-31: 12:33:25 WARNING unit.hardware-observer/31.install ImportError: cannot import name 'Literal'
unit-hardware-observer-31: 12:33:25 ERROR juju.worker.uniter.operation hook "install" (via hook dispatching script: dispatch) failed: exit status 1