Document `occ encryption:drop-legacy-filekey`

[joshtrichards]

Document `occ encryption:drop-legacy-filekey`

[joshtrichards]

Cc: @come-nc

[sorbaugh]

@come-nc consensus is to reword "Encrypting files increases their size by roughly 35%" to "Encryption files generate a slight overhead in size by ~1% (35% before Nextcloud X)"

and mention the command that can migrate it for some versions^^

https://docs.nextcloud.com/server/latest/admin_manual/configuration_files/encryption_configuration.html

[joshtrichards]

X = v25 (nextcloud/server#32110)

[AkechiShiro]

Thanks a lot to everyone involved and all the efforts into documenting and updating the encryption !