fix(GlButton): Default all dropdown button icon sizes to 16

What does this MR do?

Closes #1250 (closed). All icons in buttons and dropdown buttons should use 16 as the default size. Based on Figma designs: https://www.figma.com/file/qEddyqCrI7kPSBjGmwkZzQ/Component-library?node-id=22298%3A13380. This came up during a change to the notifications dropdown button, which contains a smaller icon than it's adjacent small button counterparts:

Before	After

Does this MR meet the acceptance criteria?

Conformity

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• GitLab UI's contributing guidlines.
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• Added the ~"component:*" label(s) if applicable.

Security

N/A

Accessibility

N/A

Closes #1250 (closed)