Drop FluentAssertions dependency

[bart-vmware]

Because FluentAssertions v8 requires a paid license for closed-source commercial usage, we'll likely migrate away from it. Although Steeltoe itself is open-source, companies that fork Steeltoe privately would need to purchase a license (per developer). This requirement reduces the attractiveness of Steeltoe, which we'd like to avoid.

Potential future steps (and accompanying tools):

• Replace with Shouldly
  • Onboarding guide for FluentAssertions shouldly/shouldly#1034 (mentions PowerShell migration script)
• Use xUnit built-in assertions
  • Migration tool at https://github.com/amoerie/fluentassertions-migrator
• Stick with FA v7 forever (or switch to a well-maintained fork, if one becomes available)
  • Pin dependency as <PackageReference Include="FluentAssertions" Version="[7.0.0,8.0.0)" /> to block package updates

Please feel free to share your thoughts and concerns regarding the license change of this dependency.