Fix 'weak variable never mutated' warnings for Xcode 16.2+

Conditionally use 'weak let' for Swift 6.2+ and 'weak var' for older versions in HeartbeatStorageTests.swift and StorageReference.swift.

Author: ncooke3

FirebaseCore/Internal/Tests/Unit/HeartbeatStorageTests.swift

@@ -64,7 +64,11 @@ class HeartbeatStorageTests: XCTestCase {
   func testCachedInstancesCannotBeRetainedWeakly() {
     // Given
     var strongHeartbeatStorage: HeartbeatStorage? = .getInstance(id: "sparky")
-    weak var weakHeartbeatStorage: HeartbeatStorage? = .getInstance(id: "sparky")
+    #if swift(>=6.2)
+      weak let weakHeartbeatStorage: HeartbeatStorage? = .getInstance(id: "sparky")
+    #else
+      weak var weakHeartbeatStorage: HeartbeatStorage? = .getInstance(id: "sparky")
+    #endif
     XCTAssert(
       strongHeartbeatStorage === weakHeartbeatStorage,
       "Instances should reference the same object."

FirebaseStorage/Sources/StorageReference.swift

@@ -316,7 +316,11 @@ import Foundation
     var prefixes = [StorageReference]()
     var items = [StorageReference]()
 
-    weak var weakSelf = self
+    #if swift(>=6.2)
+      weak let weakSelf = self
+    #else
+      weak var weakSelf = self
+    #endif
 
     var paginatedCompletion: ((_: StorageListResult?, _: Error?) -> Void)?
     paginatedCompletion = { (_ listResult: StorageListResult?, _ error: Error?) in