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.

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)