Updating existing dapp to use `openzeppelin-contracts` instead of `openzeppelin-solidity`

Support Contracts
5

Hi @dougiebuckets,

The issue is importing from both @openzeppelin/contracts-ethereum-package and @openzeppelin/contracts.

Whilst there are plans to have a single @openzeppelin/contracts library for standard and upgradeable contracts (Planning the demise of OpenZeppelin Contracts' evil twin) and to support upgradeable and non-upgradeable contracts in the same project, in the short term it might be easier to have your upgradeable contract in a separate project.