Update all `ConfigLoader` uses to `OmegaconfigLoader`

[merelcht]

Motivation and Context

While creating a demo project I noticed errors for the tests that were still referencing the old ConfigLoader.

How has this been tested?

Checklist

• [ ] Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• [ ] Assigned myself to the PR
• [ ] Added tests to cover my changes