Sidekiq log timestamps - round partial seconds to 3 digits instead of 6

What does this MR do?

Original issue: #43422 (closed)

This MR changes the sidekiq structured logging to use ISO8601 time format with 3 fraction digits, as a part of standardizing this format across all logging at Gitlab.

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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

Security

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• 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
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