Taproot channels activation check-list

[t-bast]

Now that #3118 and #3103 have been added to master, there are a few follow-up changes we should do before activating the feature:

• Add support for omitting the prevtx field in interactive-tx when splicing taproot channels, or when there is a taproot input (such as the swap-in-potentiam input for Phoenix): this will finally get rid of the issue where utxos coming from large transactions (such as coinjoins) cannot be used: see Allow omitting previousTx for taproot splices #3143
• Switch to the official feature bit and remove the staging one: see Use official feature bit for option_simple_taproot #3144