Make inter-server-sync to export/import dates in UTC (bsc#1221505)

[meaksh]

What does this PR change?

In this PR, we force ISSv1 endpoints to convert the last_modified value for synchronized objects into UTC. This prevents cache misses when the ISS master and ISS slave have different timezone config, where in the case of activating DST in different TZs, because of the different last_modified values, objects might be forced to re-sync even if it is not strictly necessary, thus increasing the ISS synchronization time.

GUI diff

No difference.

• DONE

Documentation

• No documentation needed: only internal and user invisible changes

• DONE

Test coverage

ℹ️ If a major new functionality is added, it is strongly recommended that tests for the new functionality are added to the Cucumber test suite

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• No tests: already covered

• Unit tests were added

• Cucumber tests were added

• DONE

Links

Issue(s): https://github.com/SUSE/spacewalk/issues/23977

• DONE

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[vzhestkov]

I would say we need to align the code according to check style suggestions.

[meaksh]

@vzhestkov it should be done now 👍