Amend text on pre-enforcement storage banner

What does this MR do and why?

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/350632

This amends the text on the pre-enforcement storage banner with the details mentioned in the issue. A few other MRs related with this issue but not inter-dependent:

• The ability to show this banner to sub-groups was tackled in !87804 (merged)
• The ability to show this banner on the project page is being tackled in !90811 (merged)
• The Feature Flag is being added in !90374 (merged)
• The ability to show this banner to maintainers is done in !90808 (merged)

Screenshots or screen recordings

Before
After

Root Group	SubGroup	Project

How to set up and validate locally

1. Check out this branch led/350632-update-pre-enforcement-storage-banner
2. On the terminal rails c: enable the FF: Feature.enable(:namespace_storage_limit_show_preenforcement_banner)
3. Go to a free group's page, or free subgroup's page, or a free project's page
4. Make sure the banner in the screenshot is showing

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.