Update help text for delayed group/project removal admin setting

What does this MR do and why?

Related to #389557 (closed)

Follow-up to !114503 (merged)

In the above MR we updated the UI so that group/project deletion is always delayed (behind a feature flag right now). This also involved removing the group setting. In that MR we missed updating the copy based on the proposal in the linked issue. The wording should be changed slightly and Some groups can opt-out their projects. should be removed when the feature flag is enabled because that is no longer true.

Screenshots or screen recordings

:always_perform_delayed_deletion feature flag disabled

Expected to look the same

Before	After

:always_perform_delayed_deletion feature flag enabled

Copy changed

Before	After

How to set up and validate locally

1. Enable the always_perform_delayed_deletion feature flag
   • bin/rails console
   • Feature.enable(:always_perform_delayed_deletion)
2. Go to /admin/application_settings/general and expand the Visibility and access controls section and find the Deletion protection setting.

MR acceptance checklist

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