Add TODOs to remove AccessLevelOnImport with Swift 6

Author: andrewheard

Package.swift

@@ -1310,6 +1310,8 @@ let package = Package(
       ],
       path: "FirebaseVertexAI/Sources",
       swiftSettings: [
+        // TODO(#14638): Remove "AccessLevelOnImport" experimental feature declaration after
+        // upgrading to `swift-tools-version:6.0`, where it is on by default.
         .enableExperimentalFeature("AccessLevelOnImport"),
         .enableUpcomingFeature("InternalImportsByDefault"),
       ]
@@ -1329,6 +1331,8 @@ let package = Package(
         .headerSearchPath("../../../"),
       ],
       swiftSettings: [
+        // TODO(#14638): Remove "AccessLevelOnImport" experimental feature declaration after
+        // upgrading to `swift-tools-version:6.0`, where it is on by default.
         .enableExperimentalFeature("AccessLevelOnImport"),
         .enableUpcomingFeature("InternalImportsByDefault"),
       ]