[Swift 6] Fix mutable state error in FirebaseDataEncoder.swift

FirebaseSharedSwift/Sources/third_party/FirebaseDataEncoder/FirebaseDataEncoder.swift

@@ -2611,11 +2611,14 @@ fileprivate struct _JSONKey : CodingKey {
 //===----------------------------------------------------------------------===//
 
 // NOTE: This value is implicitly lazy and _must_ be lazy. We're compiled against the latest SDK (w/ ISO8601DateFormatter), but linked against whichever Foundation the user has. ISO8601DateFormatter might not exist, so we better not hit this code path on an older OS.
-fileprivate var _iso8601Formatter: ISO8601DateFormatter = {
-  let formatter = ISO8601DateFormatter()
-  formatter.formatOptions = .withInternetDateTime
-  return formatter
-}()
+fileprivate var _iso8601Formatter: ISO8601DateFormatter {
+  // Create a new instance each time to ensure thread safety.
+  get {
+    let formatter = ISO8601DateFormatter()
+    formatter.formatOptions = .withInternetDateTime
+    return formatter
+  }
+}
 
 //===----------------------------------------------------------------------===//
 // Error Utilities