Reverse bad change to default and latest LanguageVersion

src/Compilers/CSharp/Portable/LanguageVersion.cs

@@ -332,9 +332,9 @@ public static LanguageVersion MapSpecifiedToEffectiveVersion(this LanguageVersio
             switch (version)
             {
                 case LanguageVersion.Latest:
-                    return LanguageVersion.CSharp8;
+                    return LanguageVersion.CSharp7_3;
                 case LanguageVersion.Default:
-                    return LanguageVersion.CSharp8;
+                    return LanguageVersion.CSharp7;
                 default:
                     return version;
             }