Unification of labels in global style settings to singular form

[megane9988]

What?

#56733
To resolve mixed plural and singular forms in global styling setting's labels.

Why?

I felt uncomfortable with the mixing of plural and singular forms.

How?

I have unified it into singular form.

Testing Instructions

1. Dashboard → Appearance → Editor
2. Styles → Edit styles（Pencil Icon）
3. Typography or Colors from right panel
4. Visually check that each label is unified into a singular form and that plural forms are not mixed.

Testing Instructions for Keyboard

Screenshots or screencast

[kristastevens]

Hi 👋 Thanks for the copy review ping. This looks fine!

[github-actions]

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Co-authored-by: megane9988 <megane9988@git.wordpress.org>
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[kristastevens]

Hello!

Can we have the "Needs copy review" label removed? This PR pops up in our notifications each day. Thanks!