Convert serialized date times into UTC

What does this MR do?

We should be sending UTC formatted dates to the FE when we serialize the Jira issue dates

What we get now: 2020-02-03T12:04:58.204-0800. We should be sending 2020-02-03 20:04:58 UTC

Actually, expose the utc date object (not as a string), and allow the json conversion to put the date in the right format, as we do in other cases.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #227487 (closed)