Update repository file tree row hover colors

What does this MR do and why?

Update repository file tree row hover colors

The repository file tree (tbody tr.tree-item) hover state currently uses the action-selected token family for its border, plus hard-coded gl-bg-blue-50 / dark:gl-bg-neutral-800 for the background. Per the @gitlab/ui token definitions, action-selected is reserved for persistently selected actions, while highlight-target is intended for "borders of temporarily targeted elements... distinct from persistent states". A hovered table row matches the latter.

This change:

• Replaces --gl-action-selected-border-color-default with --gl-highlight-target-border-color for the hover border and inset box-shadows.
• Replaces the hard-coded background utilities with --gl-highlight-target-background-color, which resolves to blue-50 in light mode (visually identical) and blue-950 in dark mode (intentional alignment with the hover semantic).

The hover border becomes a softer blue, and the dark-mode hover background shifts from a neutral grey to a dark blue tint.

Changelog: changed

References

Related: https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/work_items/3438

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Navigate to a repo file tree
   1. Example: http://127.0.0.1:3000/flightjs/Flight
2. Hover one of the rows

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