Put pre-enforcement storage banner behind a feature flag

What does this MR do and why?

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

This put's the pre-enforcement storage banner under a feature flag so we can control when we display the banner.

We have a few constraints to show this banner:

• User needs to be able to admin namespace
• Namespace needs to be free
• We need to have a date for when the enforcement starts
• User haven't dismissed the banner yet

So the issue here is that, at the minute we commit+deploy the enforcement date, the banner will start showing to all the users that meet the criteria above. We need a way to stop/start showing the banner, hence the feature flag.

Screenshots or screen recordings

No UI changes at this point, so just showing the banner as is:

How to set up and validate locally

1. Open your local GitLab and go to a free group
2. Make sure the banner does not appear
3. To make the banner appear:
   1. Either enable this FF: Feature.enable(:namespace_storage_limit_bypass_date_check)
   2. Or replace nil with Date.current in the storage_enforcement_date method
4. Make sure the banner does NOT appear (the FF guard clause is working)
5. Enable the FF: Feature.enable(:namespace_storage_limit_show_preenforcement_banner)
6. Make sure the banner appear

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.