fix(electrum)!: Remove `seen_at` param from `into_tx_graph`

fix(electrum)!: Remove `seen_at` param from `into_tx_graph`

and `into_confirmation_time_tx_graph`, since now it makes
sense to use `TxGraph::update_last_seen_unconfirmed`.

Also, use `update_last_seen_unconfirmed` in examples for
electrum/esplora. We show how to update the last seen
time for transactions by calling `update_last_seen_unconfirmed`
on the graph update returned from a blockchain source, passing
in the current time, before applying it to another `TxGraph`.