Audit and update buttons on Groups::BoardsController#index

What does this MR do?

• Updates the focus mode button on the Groups::BoardsController#index to use the tertiary category. This aligns with the effort in !57129 (diffs).
• The focus mode button is also hidden at the smaller breakpoints because it adds little value and it actually prevents vertical scrolling to see all of the content within a column easily.
• Change the Create list button parent to a div instead of span so that the button also gets the full width styles that the other buttons have at the smaller breakpoints.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

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Security

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Related to #273325 (closed)