polygon: switch to native usdc

[buck54321]

USDC on Polygon has only existed in the bridged form previously, but Circle is now migrating to a native token.

https://twitter.com/circle/status/1707410165769871504

Let's switch. Polygon is not in any release yet. We need to launch a contact pointing to the new mainnet address, re-generate the gas table, and provide devs with clear instructions for migration, e.g. send existing bridged to ... trade for native token by ... . I doubt any of us have a substantial amount of Polygon USDC in mainnet wallets, so I suspect this won't be too big of a deal.