Use sentence case for Admin area

What does this MR do and why?

Uses sentence case for Admin area in UI and docs

Related slack threads:

• https://gitlab.slack.com/archives/C16HYA2P5/p1720541304408439
• https://gitlab.slack.com/archives/CCB575BGT/p1720543609966749

Screenshots or screen recordings

Before	After

How to set up and validate locally

Check instances of Admin area text

Links

• ✂️ UX Paper Cuts 17.2 → Admin (#457239)