Update FluentAssertions to version 6.4.0

Author: 0xced

tests/Serilog.Formatting.Log4Net.Tests.csproj

@@ -8,7 +8,7 @@
 
   <ItemGroup>
     <PackageReference Include="coverlet.msbuild" Version="3.1.1" PrivateAssets="all" />
-    <PackageReference Include="FluentAssertions" Version="6.3.0" />
+    <PackageReference Include="FluentAssertions" Version="6.4.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.0.0" />
     <PackageReference Include="PublicApiGenerator" Version="10.3.0" />
     <PackageReference Include="ReportGenerator" Version="5.0.3" />