Update information on testnet retention policy

[luca-moser]

Under https://docs.iota.org/developer/getting-started/connect#iota-networks it is stated that: "The data on Testnet persists through the regular update process, but might be wiped when necessary. Testnet data wipes are announced ahead of time.". This isn't true, data on the Testnet must never be wiped. It is only the Devnet which can encounter data resets. Please change the section accordingly.