Nearby Voice Chat tip: fix overlay behavior or remove

### Build version
Not specified

### Issue Description
The Explorer shows a "Nearby Voice Chat" introductory tip banner to first-time users (gated via PlayerPrefs). While the tip itself is a nice first-impression touch for new users, it has two problems:

1. **Stale impression** — the feature launched months ago, so the banner can make the platform look outdated.
2. **Overlay behavior (main issue)** — if the user does not dismiss the tip, it stays on top of everything else they open (Backpack, Communities, etc.), blocking interaction with other UI panels.

### STR (Steps to Reproduce)
1. Delete the PlayerPrefs JSON file (or use a truly fresh account with no prior data) to simulate a new user.
2. Open Explorer — the Nearby Voice Chat tip appears.
3. Without dismissing the tip, open the Backpack or Communities panel.
4. Observe the tip remains rendered on top of the newly opened panel.

### Expected Result
Either:
- **Option A — Feature-flag the tip:** Add a dedicated feature flag to gate the tip independently from the Nearby Voice Chat feature itself. This lets the team turn it on/off remotely without code changes, preserving the tip for future use.
- **Option B — Fix overlay behavior:** Keep the tip but auto-dismiss or hide it when the user opens another panel (Backpack, Communities, etc.), so it never blocks other UI.
- **Option C — Remove the tip entirely:** Delete the tip flow, view, prefab, and PlayerPrefs key. The Nearby Voice Chat feature (audio pipeline, sidebar button, widget) remains unaffected.

### Actual Result with evidence
The tip banner appears and persists on top of all other UI panels until explicitly dismissed.

### Reproduction
Always (on a fresh account / cleared PlayerPrefs)

### Operative system and additional Notes
- There is a feature flag for the Nearby Voice Chat feature itself, but **not** for the tip modal/banner.
- The tip is gated via `DCLPrefKeys.NEARBY_VOICE_TIP_DISMISSED` in PlayerPrefs — it only shows once per user, but if not dismissed it stays permanently visible.
- This fits the **Discovery / First Day** scope and should be aligned with the DFD team.
- PR #9698 exists with a minimal removal approach, but the team may prefer a feature-flag or overlay-fix approach instead.

Originally reported by Esteban Ordano
Requested by Stoyan Tsekov via Slack