Replace JSON/YAML marshaling with just YAML marshaling

Original Author: @dkirillov

## Summary

The `readConfigFile` function is able to read both YAML and JSON file. Using a switch-statement it picks the correct class to unmarshal data with.

This switch statement can be removed in favor of using just YAML (from `gopkg.in/yaml.v3`) to unmarshal things.

## Motivation

Reduce and make cleaner code.

## Describe alternatives you've considered

N/A

## Additional context

An attempt was made, however, started to look like a rabbit hole with the errors that would come up (see screenshot below).

It should be possible, just would need to have a separate PR for clarity.

Locate the readConfigFile function and inspect how it currently switches between JSON and YAML unmarshaling, including the gopkg.in/yaml.v3 usage. Check the errors encountered by the earlier attempt before simplifying the logic; done means configuration files still unmarshal correctly without the switch and the related errors are resolved.

Written by the indexing model from the issue text.