Update to FluentAssertions 6.x.x

[martincostello]

Is your feature request related to a specific problem? Or an existing feature? Please describe.

Polly is currently using FluentAssertions 5.10.3, but the latest version at the time of writing is 6.3.0.

The change is a major breaking change, and simply updating the version number of the NuGet package introduces hundreds of compiler errors.

Describe your proposed or preferred solution:

Update Polly to the latest version of FluentAssertions and fix any compiler errors so it successfully builds and all the existing tests pass.

[mikkelbu]

I think I have time to take a look at this one.

[martincostello]

Fixed by #918.