Improve UI text of admin settings page - General -> Account and limit

What does this MR do and why?

Improves the UI text of the admin settings page - General -> Account and limit - so that all text complies with the GitLab Design System's criteria.

Part of #350108 (closed)

Screenshots or screen recordings

Before	After

• Screenshot 1
  • Changed 2nd sentence in help text.
• Screenshot 2
  • Change label to use (days) instead of (in days)
• Screenshot 3
  • Add periods to Learn more. links, to comply with https://design.gitlab.com/usability/helping-users/#learn-more.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Log in to GitLab as an admin.
2. On the top bar, select Menu > Admin.
3. On the left sidebar, select Settings > General.
4. Expand Account and limit.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.