license: switch server code to AGPL-3.0-or-later; keep contracts MIT

The TypeScript / HTTP-server code is now AGPL-3.0-or-later. Operators who
modify the airnode and run it as a network service must publish their
modifications under the same terms — the Affero share-back obligation is
the whole point for a project that's specifically a network service.

The Solidity contracts under `contracts/` stay MIT so consumer contracts
can inherit/integrate `AirnodeVerifier` without a copyleft obligation.

- /LICENSE — canonical AGPL-3.0 text from gnu.org
- /contracts/LICENSE — MIT covering everything under contracts/
- package.json: license = "AGPL-3.0-or-later"
- README: License section now documents the split

Author: andreogle