Bump FluentAssertions, Microsoft.NETFramework.ReferenceAssemblies and System.Threading.Tasks.Extensions

Bumps [FluentAssertions](https://github.com/fluentassertions/fluentassertions), [Microsoft.NETFramework.ReferenceAssemblies](https://github.com/Microsoft/dotnet) and System.Threading.Tasks.Extensions. These dependencies needed to be updated together.

Updates `FluentAssertions` from 7.0.0 to 8.1.1
- [Release notes](https://github.com/fluentassertions/fluentassertions/releases)
- [Changelog](https://github.com/fluentassertions/fluentassertions/blob/main/AcceptApiChanges.ps1)
- [Commits](fluentassertions/fluentassertions@7.0.0...8.1.1)

Updates `Microsoft.NETFramework.ReferenceAssemblies` from 1.0.3 to 1.0.3
- [Commits](https://github.com/Microsoft/dotnet/commits)

Updates `System.Threading.Tasks.Extensions` from 4.5.4 to 4.5.4

---
updated-dependencies:
- dependency-name: FluentAssertions
  dependency-type: direct:production
  update-type: version-update:semver-major
- dependency-name: Microsoft.NETFramework.ReferenceAssemblies
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: System.Threading.Tasks.Extensions
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

BitFaster.Caching.UnitTests.Std/BitFaster.Caching.UnitTests.Std.csproj

@@ -10,7 +10,7 @@
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
-    <PackageReference Include="FluentAssertions" Version="7.0.0" />
+    <PackageReference Include="FluentAssertions" Version="8.1.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
     <PackageReference Include="Moq" Version="4.20.70" />
     <PackageReference Include="ObjectLayoutInspector" Version="0.1.4" />

BitFaster.Caching.UnitTests/BitFaster.Caching.UnitTests.csproj

@@ -10,7 +10,7 @@
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
-    <PackageReference Include="FluentAssertions" Version="7.0.0" />
+    <PackageReference Include="FluentAssertions" Version="8.1.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
     <PackageReference Include="Moq" Version="4.20.69" />
     <PackageReference Include="ObjectLayoutInspector" Version="0.1.4" />