added holocene upgrade for unichain

Author: krofax

pages/builders/notices/holocene-changes.mdx

@@ -12,6 +12,8 @@ This page outlines breaking changes related to the Holocene network upgrade for
 If you experience difficulty at any stage of this process, please reach out to [developer support](https://github.com/ethereum-optimism/developers/discussions).
 
 <Callout type="info">
+   The Holocene upgrade for the Unichain Sepolia will be activated at **Mon Dec 18 at 00:00:00 UTC**.
+  
   The Holocene upgrade for the Sepolia Superchain will be activated at **Tue Nov 26 at 15:00:00 UTC** (`1732633200`).
 
   The Holocene upgrade for the Mainnet Superchain is optimistically scheduled for **Thu 9 Jan 2025 18:00:01 UTC**, pending governance approval.