Change a stray GRPCSendable to Sendable (grpc#1588)

Motivation:

Occurrences of GRPCSendable were replaced with Sendable in grpc#1584 as we
dropped support for 5.5. However, one was missed.

Modifications:

- Change last `GRPCSendable` to `Sendable`

Result:

Fewer warnings.

Sources/GRPC/Server.swift

@@ -453,7 +453,7 @@ extension Server {
 }
 
 extension Server.Configuration {
-  public struct CORS: Hashable, GRPCSendable {
+  public struct CORS: Hashable, Sendable {
     /// Determines which 'origin' header field values are permitted in a CORS request.
     public var allowedOrigins: AllowedOrigins
     /// Sets the headers which are permitted in a response to a CORS request.