chore: Use Ops.CollectStatus Event

[gatici]

Description

This PR uses new ops feature CollectStatus Event to manage status Charm. Event named "collect_unit_status" sets the status of charm automatically at the end of every hook.

Reference:

• https://ops.readthedocs.io/en/latest/#ops.CollectStatusEvent
• https://discourse.charmhub.io/t/how-to-set-a-charms-status/11771

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library

[gatici]

Waiting for the fix of canonical/operator#940.