Suggest updating or documenting the use of older Solidity version

[zerox-toml]

I noticed that several contracts in this repo are still using very old Solidity versions, such as pragma solidity ^0.4.16;.
Given the number of breaking changes and improvements since 0.4.x, this may lead to confusion or compatibility issues for developers working in modern environments (e.g. Solidity 0.8+).

Would you consider one of the following approaches?

1. Upgrade the examples to newer Solidity versions where relevant (especially for basic patterns that are still valid).
2. Add a note in the README or relevant folders clarifying that these are legacy examples, and specify which compiler to use for each.

This would help avoid version mismatch issues, especially when people try to compile using tools like Foundry or Hardhat which default to newer versions.