Pin FluentAssertions dependency to prevent future updates

[Asafima]

FluentAssertions v8 is now paid for commercial use, package manager should prevent any updates so that FluentResults could be used commercially.

Next phase should probably be passing all dependencies to Shouldly ,
which is now sponsored by FluentAssertions old sponsors.

Closes #231

[savornicesei]

Either move to Shouldly or replace it with it's already released fork AwesomeAssertions

[Asafima]

@savornicesei
The problem with all forks is maintenance, often most forks are not maintained, that’s why I guess Shouldly is the better option.