Update npgsql to 4.1.5.

Author: bgrainger

tests/Npgsql4.Tests/Npgsql4.Tests.csproj

@@ -6,7 +6,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.1" />
-    <PackageReference Include="Npgsql" Version="4.1.4" />
+    <PackageReference Include="Npgsql" Version="4.1.5" />
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
   </ItemGroup>