Update GlAlert spacing and remove gradient

What does this MR do?

• Decrease the inner spacing between an icon and a text for a better inner/outer space ratio.
• Remove top border gradient because this approach is not supported by any other UI element.

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

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

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Conformity

• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
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• Added the ~"component:*" label(s) if applicable.

Components

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• If the MR changes a component's API, integration MR(s) have been opened (see integrations above).

Documentation

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Security

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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.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

gitlab-org/gitlab#513105 (closed)