Set max for Zen mode textarea to screen height

What does this MR do?

• Set the max height of the Zen mode textarea to be the full screen height

Why was this MR needed?

• Max height was too short (500px)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28704 (closed)