refactor: Use ops.main instead of ops.main.main, the latter is deprecated

[DanielArndt]

This should remove the following deprecation warning:

unit-self-signed-certificates-0: 15:23:29 WARNING unit.self-signed-certificates/0.update-status /var/lib/juju/agents/unit-self-signed-certificates-0/charm/./src/charm.py:525: DeprecationWarning: Calling `ops.main.main()` is deprecated, call `ops.main()` instead

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