Pin FluentAssertions to 7.x after licence change

[emlautarom1]

Is your feature request related to a problem? Please describe.
Recently, the FluentAssertions library updated its license to a subscription model for commercial usage (see: fluentassertions/fluentassertions#2943).

Describe the solution you'd like
We need to be careful and not upgrade this dependency to versions with the new license which means using versions 7.x at most.

Describe alternatives you've considered
We could remove the library entirely or replace it with an alternative like Shouldly.

Additional context
Add any other context or screenshots about the feature request here.