Update gldeprecatedbutton to glbutton

What does this MR do?

Closes #219775 (closed)

During development, we discovered that this was not something possible with borderless icon buttons. Explained at !38763 (comment 403365983). This copies the styling workaround used in !36480 (merged), so we'll have similarity between different uses of the topbar component.

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Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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• Label as security and @ mention @gitlab-com/gl-security/appsec
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