Audit /wp-admin and sentence-case UI elements (buttons, drop-downs) for better readability

[sixhours]

I know this has been done throughout Gutenberg and in core to some extent (see #49371 and #48412). Would it be worth doing a sweep of the remainder of /wp-admin to sentence-case UI elements once and for all?

Examples:

• "Add New Post" becomes "Add new post"
• "All Categories" becomes "All categories"

The case for sentence case, h/t @danhauk:

• Sentence case improves readability by allowing users to see proper shapes of words without the breaking flow of Capital Letters (i.e. Title Case is harder to read quickly because of how we typically read using the shapes of words.)
• Sentence case respects the difference between proper nouns and the other words. For example, “Upgrade to premium plan” vs. “Upgrade to Premium Plan” could have very different meanings.
• i18n: some languages capitalize different things (e.g. all nouns are capitalized in German). Title case adds cognitive load when users have to figure out which words are nouns, for example.

I welcome your thoughts!

[sixhours]

Screenshot of filters and heading of post list in /wp-admin