Move to C# 8 consistently

Can't use most of it since it won't light up with `net461`, but at be consistent across projects. When we drop net4x later this'll take more effect.

Dapper.ProviderTools/Dapper.ProviderTools.csproj

@@ -8,7 +8,6 @@
     <TargetFrameworks>net461;netstandard2.0</TargetFrameworks>
     <SignAssembly>true</SignAssembly>
     <Nullable>enable</Nullable>
-    <LangVersion>8.0</LangVersion>
   </PropertyGroup>
   <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
     <PackageReference Include="Microsoft.CSharp" Version="4.7.0" />

Directory.Build.props

@@ -20,6 +20,8 @@
     <IncludeSymbols>false</IncludeSymbols>
 
     <PublishRepositoryUrl>true</PublishRepositoryUrl>
+    
+    <LangVersion>8.0</LangVersion>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)' == 'Release'">