[Job scheduling] Use UTC timezone with timestamps

[jedrazb]

Closes #2067

Changes

• This PR ensures are timestamps of last_access_control_sync_scheduled_at, last_incremental_sync_scheduled_at and last_sync_scheduled_at are written to ES index with explicit UTC timezone +00:00
• Job scheduling service is adapted, all references to deprecated utcnow() are replaced with now(timezone.utc), this forces the scheduling service to operate on timezoned datetime objects instead of naive system defaults
  • See good explanation by Miguel: https://blog.miguelgrinberg.com/post/it-s-time-for-a-change-datetime-utcnow-is-now-deprecated

Validation

• Added unit tests
• Manually tested, scheduled full, incremental and ac syncs, they were triggering correctly with 8.15 snapshot of the stack

Checklists

Pre-Review Checklist

• this PR has a meaningful title
• this PR links to all relevant github issues that it fixes or partially addresses
• if there is no GH issue, please create it. Each PR should have a link to an issue
• this PR has a thorough description
• Covered the changes with automated tests
• Tested the changes locally
• Added a label for each target release version (example: v7.13.2, v7.14.0, v8.0.0)

[artem-shelkovnikov]

Great catch!

[github-actions]

💔 Failed to create backport PR(s)

The backport operation could not be completed due to the following error:
There are no branches to backport to. Aborting.

The backport PRs will be merged automatically after passing CI.

To backport manually run:
backport --pr 2586 --autoMerge --autoMergeMethod squash