[NEB_224] Dashboard: Update Banner to Nebula beta launch (#6911)

<!--

## title your PR with this format: "[SDK/Dashboard/Portal] Feature/Fix: Concise title for the changes"

If you did not copy the branch name from Linear, paste the issue tag here (format is TEAM-0000):

## Notes for the reviewer

Anything important to call out? Be sure to also clarify these in your comments.

## How to test

Unit tests, playground, etc.

-->

<!-- start pr-codex -->

---

## PR-Codex overview
This PR updates the `AnnouncementBanner` component to promote the launch of "Nebula AI Beta" instead of the previous link to a payment playground.

### Detailed summary
- Changed `href` from "https://playground.thirdweb.com/connect/pay" to "https://nebula.thirdweb.com/"
- Updated `label` from "Let users pay with whatever they have without leaving your app" to "Nebula AI Beta is out and open to all"
- Modified `trackingLabel` from "ub-launch" to "nebula-beta-launch"

> ✨ Ask PR-Codex anything about this PR by commenting with `/codex {your question}`

<!-- end pr-codex -->

Author: MananTank

apps/dashboard/src/components/notices/AnnouncementBanner.tsx

@@ -47,9 +47,9 @@ function AnnouncementBannerUI(props: {
 export function AnnouncementBanner() {
   return (
     <AnnouncementBannerUI
-      href="https://playground.thirdweb.com/connect/pay"
-      label="Let users pay with whatever they have without leaving your app"
-      trackingLabel="ub-launch"
+      href="https://nebula.thirdweb.com/"
+      label="Nebula AI Beta is out and open to all"
+      trackingLabel="nebula-beta-launch"
     />
   );
 }