Pin to FluentAssertions 7.x

[bouvierr]

As mentioned in a previous issue:

Fluent Assertions 7.x is FOSS and will remain so. 7.1 was released with some nice updates and 7.2 is coming soon with bugfixes.

Originally posted by @dennisdoomen in #232 (comment)

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The nuget package for FluentResults.Extensions.FluentAssertions v2.2.1 has these dependencies:
FluentAssertions (= 6.7.0)
FluentResults (>= 4.0.0)

But we can safely change this to instead always use the latest 7.x release of FluentAssertions.

For reference, see https://fluentassertions.com/releases/#800

Version 7 will remain fully open-source indefinitely and receive bugfixes and other important corrections.

[dennisdoomen]

FWIW, open-source projects and non-profit organizations can also freely use FA 8 (like I do in all my other projects).

[Crossbow78]

A more general question is: why exactly is FluentResults.Extensions.FluentAssertions currently dependent on one specific version of FluentAssertions (= 6.7.0), rather than (>= 6.7.0)? It causes a conflict when using 7.x...

And no, let's not even consider spreading a dependency on anything beyond 7.x - even via open-source projects.

[bouvierr]

@Crossbow78 indeed, I created this issue because I have this warning:

Detected package version outside of dependency constraint: FluentResults.Extensions.FluentAssertions 2.2.1 requires FluentAssertions (= 6.7.0) but version FluentAssertions 7.2.0 was resolved.