docs: Add disclaimer to migrating-to-zksync.md (#470)

# Description

Based on a user-reported issue with `matterlabs/hardhat-zksync`, we
realized that executing `deployProxy` on non-ZKsync chains requires the
`@openzeppelin/hardhat-upgrades` package to be installed.

## Linked Issues
matter-labs/hardhat-zksync#1863

## Additional context

Author: Jrigada

content/00.zksync-network/40.tooling/20.hardhat/20.guides/20.migrating-to-zksync.md

@@ -56,13 +56,15 @@ You can learn more about each plugin in the [Getting started section](/zksync-ne
 
    - `@nomicfoundation/hardhat-ethers`: `@matterlabs/hardhat-zksync` includes `hardhat-ethers` extended with support for ZKsync and non-ZKsync networks.
    - `@openzeppelin/hardhat-upgrades`:  `@matterlabs/hardhat-zksync` includes `hardhat-upgrades` extended with support for ZKsync and non-ZKsync networks.
+     - **For multichain projects** that deploy to both ZKsync and regular Ethereum networks, `@openzeppelin/hardhat-upgrades`
+       must still be installed as the ZKsync plugin delegates to it for non-ZKsync networks.
    - `@nomicfoundation/hardhat-toolbox`: this plugin includes `@nomicfoundation/hardhat-ethers` which can cause conflicts. Import necessary
      components separately excluding `@nomicfoundation/hardhat-ethers`.
 
    ```ts
    // import "@nomicfoundation/hardhat-toolbox"
    // import "@nomicfoundation/hardhat-ethers"
-   // import "@openzeppelin/hardhat-upgrades"
+   // import "@openzeppelin/hardhat-upgrades" // Keep this import for multichain projects
     :code-import{filePath="hardhat-sol-template/hardhat.config.ts:zksync-import"}
    ```
 
@@ -395,6 +397,9 @@ The [following example project](https://github.com/uF4No/hardhat-evm-zksync-exam
 Here are some recommendations for projects that target multiple chains:
 
 - Add the desired ZKsync networks with the `zksync:true` flag to the `hardhat.config.ts`.
+- Install `@openzeppelin/hardhat-upgrades` explicitly if you're using proxy contracts and deploying to both ZKsync and regular Ethereum networks.
+  The `@matterlabs/hardhat-zksync` plugin uses its own implementation for ZKsync networks but delegates to OpenZeppelin's implementation
+  for non-ZKsync networks.
 - Make sure to run the compilation task with the `--network` flag when targeting ZKsync networks to use the custom compiler.
 - When targeting ZKsync chains, the `@matterlabs/hardhat-zksync` plugin overrides the following plugins: `@nomiclabs/hardhat-ethers`, `@openzeppelin/hardhat-upgrades`.
 - To avoid typescript collision errors between `@nomiclabs/hardhat-ethers` and