fix: re-export UNSTABLE_useLoadMoreSentinel so it isn't a breaking change

[LFDanLu]

Closes #8635

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

🧢 Your Project:

RSP

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## API Changes

@react-aria/utils

/@react-aria/utils:useLoadMoreSentinel

-useLoadMoreSentinel {
-  props: LoadMoreSentinelProps
-  ref: RefObject<HTMLElement | null>
-  returnVal: undefined
-}

/@react-aria/utils:UNSTABLE_useLoadMoreSentinel

+UNSTABLE_useLoadMoreSentinel {
+  props: LoadMoreSentinelProps
+  ref: RefObject<HTMLElement | null>
+  returnVal: undefined
+}

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