Remove blue from buttons and prepare for tints

What does this MR do?

Update buttons to only use neutral and danger colors. This reduces overall color use in the UI and enables buttons to be more responsive to theme changes (see theme approach). This is a change that's been explored even prior to dark mode exploration and featured predominantly in recent Project Sneak Peek work.

Alpha neutrals

https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/work_items/3429+ introduces tinted neutrals. This MR also replaces some hard coded alpha values with color.alpha.light/dark.* so that they can inherit the tints later.

The previous values used colored-tint overlays (e.g. neutral-500 at 16%), which when swapped 1:1 to neutral-black/white alpha tokens at the same step produced visibly heavier hover/focus/active backgrounds — most noticeably on dropdown options and confirm-secondary buttons. To preserve the original visual weight, the alpha steps were re-mapped to closer perceptual matches (e.g. neutral-500 at 16% → alpha.dark.6 on hover, neutral-700 at 24% → alpha.dark.16 on active).

Once tinted neutrals land in #3429, these tokens will automatically pick up the brand tint without further changes.

Additional considerations

• Secondary buttons are in the process of being removed. ​ #2013
• Tertiary confirm and tertiary default have the same styles, the plan is for one to be removed.
• There are many screenshots in the product docs and design system docs that need to be updated based on this.
• New interactive color rules need to be documented to replace https://design.gitlab.com/product-foundations/color/#interactive-states

Screenshots or screen recordings

Before	After

Integrations

• GitLab: mr_url

• CustomersDot: mr_url

• Duo UI: mr_url

• Status Page: mr_url

• Docs: mr_url

• Switchboard: mr_url

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Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
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Related to #3434