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.

Inspect the Context sections of Echidna exercises 5, 6, 7, and 8, then verify which Damn Vulnerable DeFi version those exercises target. Update the links from the current v4 reference to the compatible older version and confirm that all four exercise references are consistent.

Written by the indexing model from the issue text.