Homepage button docs should say the image should be 24 pixels high

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA

What does this MR do and why?

This fixes the documentation. On the GitLab Appearance page, subsection Customize your homepage button, it states: The image should be 28 pixels high. Images more than 28 px high will be resized.

I believe this is in error and it should state: The image should be 24 pixels high. Images more than 24 px high will be resized.

GitLab itself states 24 pixels when you go to update the homepage button here:
https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/views/admin/application_settings/appearances/_form.html.haml?ref_type=heads#L61

In GitLab on the page to update the header logo it says: Maximum file size is 1MB. Pages are optimized for a 24px tall header logo

Looking at our custom header logo on our self-hosted instance I can see it's 24px as well

Screenshots or screen recordings