feat: use inventory's check_in_time to extract and list offline devices

[tranchitella]

Ticket: MEN-7251
Changelog: Title

[mender-test-bot]

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[mzedel]

lgtm - just the failing unit test left. let me know if you want to offload this 👌

[tranchitella]

lgtm - just the failing unit test left. let me know if you want to offload this 👌

Can you please provide me with guidance on how to fix this failing unit test?
Unfortunately, my skills are not good enough to understand why this is failing.

[mender-test-bot]

Merging these commits will result in the following changelog entries:

Changelogs

gui (men-7251)

New changes in gui since master:

Features

• use inventory's check_in_time to extract and list offline devices
  (MEN-7251)

[mender-test-bot]

Hello 😺 This PR contains changelog entries. Please, verify the need of backporting it to the following release branches:
3.7.x (release 3.7.x)
3.6.x (release 3.6.x)