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 (closed) #424387 (closed)

EE: true

No changelog since this is behind a feature flag.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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