Polish dropdown trigger padding

What does this MR do?

This MR updates the new dropdown trigger to match the design spec in the UI kit.

• When the down caret is visible, always have reduced right padding for optical alignment.
• When the label is visible, x-axis padding appears bigger than y-axis padding.
• For icon only triggers, y- and x-axis padding appears even.

Out of scope: Deprecating and or updating the dropdown button that is part of the GlButton component.

Screenshots or screen recordings

Before	After

Bonus

Here's the rich text editor doing its thing

After

Integration merge requests

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• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

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Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

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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.

Closes #2430 (closed)