Let the verification dialog figure out on its own whether you're using a recovery key or recovery passphrase

[duxovni]

(Splitting this off from #19279 , which covered several independent UI bugs found while reviewing matrix-org/matrix-react-sdk#6811)

When we're setting up cross-signing on a new device using the key backups in secret storage, it's awkward to only prompt for the recovery passphrase, and have a separate link to click if you want to use the recovery key instead. Ideally this would just be a single textbox that can take either a passphrase or a key and figure out which one it is, alongside a button for uploading a key instead.