docs: Add migration notice to README (#354)

Updated README to reflect migration to core monorepo and archiving of
the repository.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Updates README to indicate the repo is archived and future development
occurs in the core monorepo.
> 
> - **Documentation**:
> - Update notice in `README.md` to reflect completed migration,
repository archival, and redirect future development to `core` monorepo.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
7017531. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: Mrtenz

README.md

@@ -1,6 +1,20 @@
 # @metamask/eth-block-tracker
 
-<table><tr><td><p align="center"><b>⚠️ PLEASE READ ⚠️</b></p><p align="center">This package is currently being migrated to our <a href="https://github.com/MetaMask/core"><code>core</code></a> monorepo. Please do not make any commits to this repository while this migration is taking place, as they will not be transferred over.</p></td></tr></table>
+<table>
+  <tr>
+    <td>
+      <p align="center"><b>⚠️ PLEASE READ ⚠️</b></p>
+      <p align="center">
+        This package has been migrated to our
+        <a href="https://github.com/MetaMask/core"><code>core</code></a>
+        monorepo, and this repository has been archived. Please note that all
+        future development and feature releases will take place in the
+        <a href="https://github.com/MetaMask/core"><code>core</code></a>
+        repository.
+      </p>
+    </td>
+  </tr>
+</table>
 
 This module walks the Ethereum blockchain, keeping track of the latest block. It uses a web3 provider as a data source and will continuously poll for the next block.