Improve Roadmap today indicator rendering logic

What does this MR do?

This MR updates logic for rendering timeline today indicator in Roadmap (that red vertical bar) such that we don't have to fiddle around with z-index or use JS scroll event listener to toggle its visibility when timeline is scrolled horizontally (i.e. better ~performance while scrolling timeline horizontally).

Screenshot

Before	After

Notice how timeline bars now appear on top of indicator, which wouldn't have been possible otherwise.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
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• 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

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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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