Update to Npgsql 7.0.

Author: bgrainger

tests/Npgsql7.Tests/Npgsql7.Tests.csproj

@@ -10,7 +10,7 @@
     <Using Include="Npgsql" />
     <Using Include="Xunit" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.4.0" />
-    <PackageReference Include="Npgsql" Version="7.0.0-rc.2" />
+    <PackageReference Include="Npgsql" Version="7.*" />
     <PackageReference Include="xunit" Version="2.4.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5" />
   </ItemGroup>