os-nextcloud-backup Add some note about UTC time, in case people get confused:)

[Nuadh123]

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Related issue
#5327

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Describe the problem
Files named YYYYMMDD is timezone agnostic, but the code isn't.. UTC is the truth here. Users might get confused about this, when noone say it is so.

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Describe the proposed solution
Add text describing that UTC is the timezone utilized for naming.

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