CSHARP-4923: Upgrade to C# v12 (#1490)

Author: BorisDog

src/Directory.Build.props

@@ -5,7 +5,7 @@
 
   <PropertyGroup>
     <TargetFrameworks>netstandard2.1;net472;net6.0</TargetFrameworks>
-    <LangVersion>10.0</LangVersion>
+    <LangVersion>12.0</LangVersion>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <CodeAnalysisRuleSet>..\..\MongoDB.ruleset</CodeAnalysisRuleSet>
   </PropertyGroup>