Upgrade Contracts to storage-plus

[ethanfrey]

Let's get them all using this for consistency. When you work to refactor/extend a contract that still uses cosmwasm-storage, just update it and check a box here.

Contracts to update:

• cw1-subkeys
• cw1-whitelist
• cw3-fixed-multisig
• cw3-flex-multisig
• cw4-group
• cw4-stake
• cw20-atomic-swap
• cw20-base
• cw20-bonding
• cw20-escrow
• cw20-staking
• cw721-base