Upgrade Solidity to 0.9.0 and remove legacy transfer/send methods

## 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.