fix(GlPagination): Remove size options - default to md

What does this MR do?

Closes #1039 (closed). This removes the size option from GlPagination. We do not use the sm or lg size in Pajamas, so this removes the size options and defaults the size to md. Based on the GlPagination > Figma audit issue.

Does this MR meet the acceptance criteria?

Conformity

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

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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Closes #1039 (closed)