[Feature] Remove old files asynchronously after update

[pludi]

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Feature request

Which Nextcloud Version are you currently using: 29.0.9
Is your feature request related to a problem? Please describe.
There is unnecessary additional downtime during an update between steps 8 and 11 due to waiting for the updater to remove old files.
Describe the solution you'd like
Instead of removing the old files in sequence with all other steps, move them off to a dedicated directory (similar to the backup step) and add a single-shot cron job to remove that directory later. Since on most Unix-like systems a move of a directory on the same filesystem is much faster than traversing it depth-first for removal this could cut down time spent in Maintenance mode.