Update runtime compilation to target CSharpLang v8.0 by default (#13110)

Fixes #9129

Author: pranavkm

src/Mvc/Mvc.Razor.RuntimeCompilation/src/CSharpCompiler.cs

@@ -211,10 +211,8 @@ private static CSharpParseOptions GetParseOptions(
 
             if (string.IsNullOrEmpty(dependencyContextOptions.LanguageVersion))
             {
-                // During preview releases, Roslyn assumes Preview language version for netcoreapp3.0 targeting projects.
-                // We will match the behavior if the project does not specify a value for C# language (e.g. if you're using Razor compilation in a F# project).
-                // Prior to 3.0 RTM, this value needs to be changed to "Latest". This is tracked via https://github.com/aspnet/AspNetCore/issues/9129
-                parseOptions = parseOptions.WithLanguageVersion(LanguageVersion.Preview);
+                // If the user does not specify a LanguageVersion, assume CSharp 8.0. This matches the language version Razor 3.0 targets by default.
+                parseOptions = parseOptions.WithLanguageVersion(LanguageVersion.CSharp8);
             }
             else if (LanguageVersionFacts.TryParse(dependencyContextOptions.LanguageVersion, out var languageVersion))
             {