[Feature flag] Enable Incident Management On-call Schedules [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

The smallest MVC possible for: #283914 (closed). This merge is tied heavily to: !55161 (merged) (!56263 (comment 527705000)).

As above, this merge was not heavily tied too: !55161 (merged) after some investigation!

Screenshots (strongly suggested)

N/A

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
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  • I have not included a changelog entry because _____.
• [-] Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
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