Restore correct additional CSS screen layout

[t-hamano]

• Related to Global Styles: Add UI package #72599
• Fixes Additional CSS: Input misplaced and not full-height #73128

What?

This PR adds the following changes to restore the correct additional CSS screen layout:

• Remove the "Additional CSS" navigation button. This was an intentional decision made in Site Editor: make less prominent Addtional CSS UI #71550.
• Restore the correct label and description.
• Make the textarea full-height.

Why?

In my research, this is a regression that occurred in #72599.

How?

Testing Instructions

• Access the site editor > Styles
• Make sure there is no "Additional CSS" navigation button
• Access the "Additional CSS" screen from the drop-down button in the top right.
• Confirm the layout.

Before	After

[github-actions]

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Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: youknowriad <youknowriad@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: jasmussen <joen@git.wordpress.org>

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[youknowriad]

Thanks for the fix, I appreciate it.

[youknowriad]

I tested both global styles top level and within the editor, both work. Thanks :)

[Mamaduka]

Noticed the following warning when opening the Styles panel, not 100% sure if it's related to this PR.

Screenshot