Upgrade and pin Fluentassertions to latest free version

[EluciusFTW]

Fixes #245

In this commit, the FluentAssertions version was pinned to 6.7.0.

This prevents many clients to upgrade to FR 4.0, if they use FA 7.x and especially the helper package FR.Extensions.FluentAssertions, since a breaking change in FR needs an update of that, which they can't do. See this issue for more details.

This PR upgrades and pins FA to the latest release (7.2.0) not concerned by the new commercial license model of FA.