feat: Add gl-z-index-200 util class

What does this MR do?

Adds the gl-z-index-200 class.

This is used in gitlab!73725 (merged), where we want to set the <aside> element to a z-index value that's higher than the pipeline icons (which can vary in the 90s-100s values since they overlap each other) but lower than the modal z-index so it doesn't overlap the modal or the menu dropdown.

See gitlab!73725 (comment 737345614) for context.

Does this MR meet the acceptance criteria?

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