replace dead Grove RPC endpoint for arbitrumMainnet

[Kajko25]

Closes #284.

arbitrumMainnet points at https://arbitrum-one.rpc.grove.city/v1/01fdb492, and that hostname no longer exists — both Cloudflare (1.1.1.1) and Google (8.8.8.8) return NXDOMAIN for it, so hardhat --network arbitrumMainnet fails at name resolution before it ever reaches a node. grove.city itself still resolves, so this is the endpoint being gone rather than the provider being down.

The issue anticipated this as something to fix "before Grove's RPC service is discontinued" — as of today it has already happened.

Replaced with Arbitrum's official public endpoint. Rationale for that specific one:

• Consistent with the neighbouring entry. arbitrumSepolia, two blocks above, already uses the official https://sepolia-rollup.arbitrum.io/rpc.
• Verified against this config. eth_chainId on https://arb1.arbitrum.io/rpc returns 42161, matching the chainId this network block declares. (The sibling Sepolia endpoint likewise returns 421614, matching its own block.)
• No third-party account id. The old URL embedded the portal id 01fdb492, which pointed every clone of this public repo at one account's quota with a provider none of the other networks here use. Grove appears exactly once in the repository, so nothing else depends on it.

One-line change; no other network entries touched.