Clarify sepolia and mainnet timestamps

Co-authored-by: soyboy <[email protected]>

Author: bradleycamacho

pages/notices/upgrade-15.mdx

@@ -106,7 +106,7 @@ These following steps are necessary for every node operator:
 <Steps>
   ### Update to the latest release
 
-  The releases are for the Isthmus Mainnet Superchain activation.
+  The releases contain both the Isthmus Mainnet and Sepolia Superchain activation timestamps.
 
   *   [`op-node` at `v1.13.2`](https://github.com/ethereum-optimism/optimism/releases/tag/op-node%2Fv1.13.2)
   *   [`op-geth` at `v1.101503.4`](https://github.com/ethereum-optimism/op-geth/releases/tag/v1.101503.4)