Migrate from FluentAssertions to Shouldly

[Joy-less]

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As stated in #661, FluentAssertions v8 changed their license from open source (Apache 2.0) to paid costing $130 per seat for commercial use.

This pull request migrates the tests in BitFaster.Caching to Shouldly, a similar library under the BSD-3-clause license.

Alternatively, you can continue using FluentAssertions without updates, but you must pin it at [7.0.0] or [7.1.0] to prevent accidental updates.

There was a huge amount to change here so I used this PowerShell script to help me out: galadril/Shouldly.FromFluentAssertions