fix(firebase_auth, ios): add missing uppercase W to nonce character set (#18775)

- The character set in `randomNonce` skipped `W` (...UVXYZ)
- Generation uses rejection sampling against the set size, so the extra
  character introduces no bias

Author: masteradit

packages/firebase_auth/firebase_auth/ios/firebase_auth/Sources/firebase_auth/FLTFirebaseAuthPlugin.swift

@@ -214,7 +214,7 @@ public class FLTFirebaseAuthPlugin: NSObject, FlutterPlugin, FLTFirebasePluginPr
 
   func randomNonce(_ length: Int) -> String {
     precondition(length > 0)
-    let characterSet = Array("0123456789ABCDEFGHIJKLMNOPQRSTUVXYZabcdefghijklmnopqrstuvwxyz-._")
+    let characterSet = Array("0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-._")
     var result = ""
     var remainingLength = length
     while remainingLength > 0 {