Fix warnings from existential and deprecation (#3800)

* Use explicit 'any' for existential.

* Fix deprecation warning.

Author: mbrandonw

Sources/ComposableArchitecture/Effect.swift

@@ -423,7 +423,7 @@ extension Effect {
       name: String?,
       priority: TaskPriority? = nil,
       operation: @escaping @Sendable () async throws -> Success
-    ) where Failure == Error {
+    ) where Failure == any Error {
       self.init(priority: priority, operation: operation)
     }
   }

Sources/ComposableArchitecture/Macros.swift

@@ -56,6 +56,7 @@ public macro Reducer(state: _SynthesizedConformance..., action: _SynthesizedConf
 #endif
 public struct _SynthesizedConformance: Sendable {}
 
+@available(*, deprecated, message: "Define your conformance using an extension, instead")
 extension _SynthesizedConformance {
   /// Extends the `State` or `Action` types that ``Reducer()`` creates with the `Codable`
   /// protocol.