Update version reference for Damn Vulnerable DeFi challenges in Echidna exercises

The link provided in the **Context** section of Echidna exercises 5, 6, 7, and 8 currently points to the latest version of Damn Vulnerable DeFi (v4). 

However, these exercises are written for an older version of the Damn Vulnerable DeFi project, which has since been updated. As a result, users may encounter discrepancies or compatibility issues when following the exercises.
