Enable ExistentialAny for v2 (#1967)

Motivation:

The use of existentials can be made more obvious by enabling
the upcoming 'ExistentialAny' feature.

Modifications:

Enable 'ExistentialAny' and fix warnings.

Result:

Use of existentials is more obvious

Tests/GRPCProtobufTests/ProtobufCodingTests.swift

@@ -92,7 +92,7 @@ struct TestMessage: SwiftProtobuf.Message {
     return true
   }
 
-  func isEqualTo(message: SwiftProtobuf.Message) -> Bool {
+  func isEqualTo(message: any SwiftProtobuf.Message) -> Bool {
     return false
   }
 }