fix csharp highlighting (#12321)

Author: bruno-garcia

docs/platforms/dotnet/common/migration/index.mdx

@@ -171,7 +171,7 @@ The format of the stack traces were greatly improved with the help of Ben Adam's
 
 However, this will **affect grouping**. If you prefer to stay with the original stack trace format, you can opt-out of this feature with:
 
-```
+```csharp
 options.StackTraceMode = StackTraceMode.Original;
 ```
 
@@ -199,7 +199,7 @@ Setting an instance of `Exception` that was caught in a `catch` block could fail
 
 To avoid this issue, pass a serializable object into Sentry. For example, map the instance of `Exception` to an anonymous object:
 
-```
+```csharp
 try
 {
     throw new ArgumentNullException("test");
@@ -229,7 +229,7 @@ ASP.NET (**not Core**) users need to install an additional package, `Sentry.AspN
 
 You can plug the package into the `Init` as follows:
 
-```
+```csharp
 options.AddAspNet();
 ```