Replace gen_event with logger_filter

[alanj853]

Elixir 1.15 brought in a lot of changes to logging, particularly around its use of :error_logger. Reports were no longer being received in Shoehorn.ReportHandler. To get around this, we set up a logger_filter that checks for application starts and stop, handles them if necessary, and then lets the other logger filters work as they normally would.

We also make Shoehorn.ReportHandler a GenServer for 2 reasons:
1. To handle the state
2. If the user was to run an Application call in their callback, they will get a crash about a process calling itself. So the call must be done from a separate process because we will get errors about trying to start an application

Lastly, bump up the min Elixir version

[fhunleth]

@alanj853 Thanks! This is on my todo list. I'll be back soon.