Fix text about Ultimate features and clarify config page

What does this MR do and why?

This MR adds some clarifying text to the Security & Compliance -> Configuration page to address #375745 (closed).

Additionally, this MR removes Container Scanning from the list of Ultimate features in the banner below. Container scanning was made available in the Free tier in the GitLab 15.0 release.

What are the relevant issue numbers?

Security Configuration enablement is confusing ... (#375745 - closed) • Unassigned • 15.5

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