Add banner message for pipeline maintenance mode

What does this MR do and why?

Add banner message for pipeline maintenance mode

• We are transitioning to security scan execution polcies and want users to be aware of this upcoming change.

• This is the first part of these changes, the edit form for frameworks will be done in a following MR.

• Refs: #441352 #440149 #424387

EE: true

No changelog since this is behind a feature flag.

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable FF: run rails c then enter Feature.enable :ff_compliance_pipeline_maintenance_mode
2. Visit Compliance center for a top level group with Ultimate license
3. Go to Frameworks tab
4. You should see the new banner

Remaining work

• only show banner when feature flag is enabled
• fix specs
• add correct paths to both links contained in the warning
• spell check