LangVersion bump

Author: danielmarbach

projects/RabbitMQ.Client.OAuth2/RabbitMQ.Client.OAuth2.csproj

@@ -31,7 +31,7 @@
       https://learn.microsoft.com/en-us/answers/questions/1371494/for-net-standard-2-0-library-why-add-net-core-3-1
       https://devblogs.microsoft.com/dotnet/embracing-nullable-reference-types/#what-should-library-authors-do
     -->
-    <LangVersion>9.0</LangVersion>
+    <LangVersion>12.0</LangVersion>
     <Nullable>enable</Nullable>
   </PropertyGroup>