Replace Moq dependency with a different Mocking library

[bacherfl]

Due to the controversies around the Moq library (see https://github.com/moq/moq/issues/1372), we may want to consider switching to another mocking library, like e.g. NSubstitute to avoid any privacy issues.
[1] Contains an example of how this was done by another previous Moq user.

Note: Currently we are using v4.18 of Moq, and the controversial change has been introduced with v4.20 and above, so currently this repo is not directly affected by the change. However, we should not merge any renovate PRs bumping the Moq dependency

References:
[1] Afterlife-Guide/AppSettings.Merge#6