revert back mongodb sink due to test failures on net6

Author: followynne

tests/Serilog.Ui.MongoDbProvider.Tests/Serilog.Ui.MongoDbProvider.Tests.csproj

@@ -10,7 +10,7 @@
 		<PackageReference Include="EphemeralMongo6.runtime.linux-x64" Version="1.0.0" Condition="$([MSBuild]::IsOSPlatform('Linux'))" />
 		<PackageReference Include="EphemeralMongo6.runtime.osx-x64" Version="1.0.0" Condition="$([MSBuild]::IsOSPlatform('OSX'))" />
 		<PackageReference Include="EphemeralMongo6.runtime.win-x64" Version="1.1.3" Condition="$([MSBuild]::IsOSPlatform('Windows'))" />
-		<PackageReference Include="Serilog.Sinks.MongoDB" Version="6.0.0" />
+		<PackageReference Include="Serilog.Sinks.MongoDB" Version="5.4.1" />
 	</ItemGroup>
 
 	<ItemGroup>