Increase the maximum allowed length for Namespace::Detail#description

What does this MR do and why?

This aligns the validation with Project#description to avoid invisible errors that prevent project deletion or transfer.

References

• Related to Unable to delete project if description length ... (#556328 - closed) • Rémy Coutable • 18.3.
• Related to https://gitlab.com/gitlab-com/request-for-help/-/issues/3142+s

How to set up and validate locally

1. Set a project's description to 501 characters (through the Rails console).
2. Try to delete the project: you should get a Project could not be updated! without the changes from this MR, and no error with the changes from this MR.
3. Try to transfer the project to another namespace: you should get a 422 The change you requested was rejected without the changes from this MR, and no error with the changes from this MR.

MR acceptance checklist

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