Replace useRefObjectAsForwardedRef with useMergedRefs internally

[llastflowers]

recreate: #7638

revert-revert: #7737

Closes: https://github.com/github/primer/issues/6787

After #7672 deprecates useRefObjectAsForwardedRef, we need to replace internal calls with the new approach, which has several advantages:

1. Fixes the bug identified in Revert "perf(useRefObjectAsForwardedRef): add dependency array to useImperativeHandle" #7635 (comment) where callback refs would be called on every render, by using a callback ref itself as the trigger for updating the refs. This prevents possible infinite render loops where a callback ref is used to set state.
2. Using a callback ref, only updates the refs when the target changes, gaining the perf improvement from perf(useRefObjectAsForwardedRef): add dependency array to useImperativeHandle #7553 without the bug that came with it.
3. Doesn't care about the order of passed arguments; the current hook will silently fail to work if you switch the arguments.
4. Can work with two callback refs, allowing for components to use callback refs internally while still accepting external refs.
5. Is React 19 forward-compatible with callback ref cleanup functions while remaining backwards-compatible with React 18.
6. Has better type definitions to reduce the need for casting.
7. Accepts undefined, a significant DX improvement for using optional React 19 ref props without having to default to null.

However, the new hook is a breaking change from the old one because it returns a combined ref that must be passed to the underlying child component. So I've still left the old hook in place (because it's public) but deprecated it so we can remove it in a future major release.

Internally, we can migrate everything over to the new approach now, which is what I've done in this PR.

• Stacks on Update useMergedRefs to be React 19 compatible and public + deprecate useRefObjectAsForwardedRef and useProvidedRefOrCreate #7672

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

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🦋 Changeset detected

Latest commit: b18bf21

The changes in this PR will be included in the next version bump.

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[github-actions]

⚠️ Action required

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[primer]

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