Use C# 8.0.

Author: bgrainger

src/Directory.Build.props

@@ -13,7 +13,7 @@
     <RepositoryType>git</RepositoryType>
     <RepositoryUrl>https://github.com/mysql-net/MySqlConnector.git</RepositoryUrl>
     <DebugType>embedded</DebugType>
-    <LangVersion>latest</LangVersion>
+    <LangVersion>8.0</LangVersion>
     <PublishRepositoryUrl>true</PublishRepositoryUrl>
     <EmbedUntrackedSources>true</EmbedUntrackedSources>
   </PropertyGroup>