fix: Align with design and docs

What does this MR do?

• Updates combobox examples so that "Select all" isn't available in the footer.
• Updates header action text from "Select All" to "Select all" to match correct casing.

Screenshots or screen recordings

Integration merge requests

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

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Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Security

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Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.