Clarify that turning on the `can_create_group` setting affects only new users

What does this MR do and why?

This MR includes a minor edit to the text in the UI to clarify that turning this setting on or off ONLY applies to users signing up/created from then on. This setting does not affect the ability of existing users to create projects.

Clarifying the detail via this text avoids a possible source of confusion.

No changelog added as this only includes an edit to UI text.

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