Explicitly use latest Serilog version (2.10.0) in the tests

Author: 0xced

tests/Serilog.Formatting.Log4Net.Tests.csproj

@@ -13,6 +13,7 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.11.0" />
     <PackageReference Include="PublicApiGenerator" Version="10.2.0" />
     <PackageReference Include="ReportGenerator" Version="4.8.13" />
+    <PackageReference Include="Serilog" Version="2.10.0" />
     <PackageReference Include="Serilog.Enrichers.Environment" Version="2.2.0" />
     <PackageReference Include="Serilog.Enrichers.Thread" Version="3.1.0" />
     <PackageReference Include="Verify.Xunit" Version="13.3.0" />