Use text.color.link for hover color in issuable sidebar controls

What does this MR do and why?

Use text.color.link for hover color in issuable sidebar controls

• Update GlButton class overrides
• Update .right-sidebar a:not(.btn) hover CSS styles

References

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MR acceptance checklist

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Screenshots or screen recordings

note: colors that don't update between color modes are not using design tokens and therefore require seperate stylesheets for light and dark mode to compile SCSS variables.

Before	After
CleanShot_2025-03-04_at_16.44.10	CleanShot_2025-03-04_at_16.42.38

How to set up and validate locally

1. Open MR view https://gdk.test:3443/flightjs/Flight/-/merge_requests/1/
2. Review "assign yourself" buttons hover styles in light and dark mode
3. Review other link sidebar actions hover color

Related to #523029 (closed)