fix(GlDropdown): apply proper classes for secondary category

What does this MR do?

This ensures that GlDropdown's toggle's classes are set properly.

A change introduced in !1645 (merged) is preventing the secondary category from being properly handled in GlDropdown. This MR fixes that by aligning the classes being applied on the dropdown's toggle with the ones defined in GlButton's styles.

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
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• 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:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
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