[lib][web][landing] Update to RainbowKit 0.12

Summary:
RainbowKit 0.12 uses Wagmi 0.12, which uses ethers.js under the hood.

RainbowKit 1.0 has been released, which uses Wagmi 1.0. Wagmi 1.0 switches from ethers.js to Viem, which results in some significant changes. Additionally, RainbowKit 0.12 supports WalletConnect 2.0, which is the main goal of this work. As a result, I'm deferring the RBK / Wagmi 1.0 upgrades for now.

Test Plan:
I tested all three SIWE endpoints:

1. Web
2. Native (old flow)
3. Native (new flow)

For each, I tested both Rainbow and WalletConnect (both of which need WalletConnect), and navigated around the WalletConnect modal to various places.

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8250