Update json-rpc.md to make Devnet the default

[Clay-Mysten]

No description provided.

[Romainua]

@Clay-Mysten what about join incentivized Testnet is it actual if validator registration is now closed? We can add it when will open or add comment like if form is close, waiting a new way of incentivized Testnet, what do you think?

[Clay-Mysten]

@Clay-Mysten what about join incentivized Testnet is it actual if validator registration is now closed? We can add it when will open or add comment like if form is close, waiting a new way of incentivized Testnet, what do you think?

Great point, Roman. I have removed the link and am asking internally if we should also update the blog post:
https://sui.io/resources-sui/announcing-sui-incentivized-testnet/

Thanks!