Fix public mumbai RPC

[vmichalik]

Chainstack have sunset their Mumbai RPC. Added latest public RPC from the Polygon wiki.

Description

Replaces deprecated Mumbai RPC and default

Additional Information

• I read the contributing docs (if this is your first contribution)

Your ENS/address: vijay.eth

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Latest commit: 3cff1de

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[tmm]

Chains should now be edited/added on the Viem repository.