Update csproj C# language version

Author: scorteanucosmin

src/Oxide.CSharp.csproj

@@ -15,7 +15,7 @@
     <TargetFrameworks>netstandard2.0;netstandard2.1;net48</TargetFrameworks>
     <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
     <NoWarn>NU1701</NoWarn>
-    <LangVersion>13</LangVersion>
+    <LangVersion>14</LangVersion>
     <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>