Upgrade Solidity to 0.9.0 and remove legacy transfer/send methods

[hadv]

Description

Upgrade the codebase to use Solidity version 0.9.0.
This involves updating the pragma solidity in all .sol files and updating the compiler configuration.

Requirements

• Update pragma solidity versions in all src/ files to 0.9.0.
• Update foundry.toml or other config files if necessary.
• Ensure transfer() and send() are not used (preliminary search showed no usage).
• Verify all tests pass with the new version.