"Upgrade your encryption" flow is broken and should be removed

[richvdh]

If we detect that you have key backup, but no 4S, we pop up a toast prompting you to "Upgrade your encryption".

This is broken in several ways:

• We also show the toast in the situation that you have 4S, but the backup key is not stored in 4S [at least, not under the current 4S key]: see Key Backup Failing to proceed. #27100 (comment). This is not a situation we can "upgrade" from.
• It produces errors like Error bootstrapping secret storage createSecretStorageKey() callback did not return a secret storage key which then produces obscure UI feedback like "Unable to set up secret storage" (Unable to setup secret storage element-desktop#1910)
• It doesn't report any failures correctly (see Migrating key backup to 4s fails #27383).
• It's not tested (as is evident from the catalogue of fail above)

Frankly, I don't think this is a flow we should be supporting at all. We should remove the "upgrade your encryption" flow.

[richvdh]

After discussion with the team today: we're agreed that this flow should be removed altogether. When logging into an account that has key backup but the keys for that backup are not available in 4S, the only option will be to create a new key backup.

[richvdh]

If we're removing the "upgrade" flow, we should also remove the behaviour in RestoreKeyBackupDialog which prompts you to enter your backup key directly (referring to it, confusingly, as "security key"). Related: #27469

[crjo]

Hi there, every new user on our on prem instance is unable to have a correct key backup at first login, is there a workaround ?
It's not occasionally for us, it's every time.

[florianduros]

@richvdh So what do we need to do "If we detect that you have key backup, but no 4S" instead of showing this broken flow?

[richvdh]

Treat it the same as having no 4S (and no key backup), imho

[florianduros]

Done #28290 & #28299