Update setup.mdx (#644)

The soroban-cli version in this doc is outdated and will give the following error when invoking a deployed contract:

"error: xdr processing error: xdr value invalid"

As specified here an update of the CLI version is needed:

https://stellar.org/blog/developers/protocol-21-upgrade-guide

docs/smart-contracts/getting-started/setup.mdx

@@ -84,7 +84,7 @@ The [Soroban CLI](http://github.com/stellar/soroban-cli) can execute Soroban con
 Install the [latest released version](https://github.com/stellar/soroban-cli/releases) of Soroban CLI using `cargo install`.
 
 ```sh
-cargo install --locked soroban-cli
+cargo install --locked soroban-cli --version 21.0.0-rc.1
 ```
 
 :::info