Add maintain_namespace policy

What does this MR do and why?

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

This creates a maintain_namespace policy so that we can show the pre-enforcement storage banner for both owner and maintainer roles.

Screenshots or screen recordings

Here's a video demonstrating the banner appearing to any group for admin user, and the groups where a regular user is maintainer or owner

maintain_namespace_policy

How to set up and validate locally

1. 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
2. Go to a free group's page, or free subgroup's page
3. Make sure the banner in the vide is showing for the group's you're either owner or maintainer and not showing for other roles

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.