VACMS-19471: Alternative banner feature toggle

[dsasser]

Description

Adds a feature toggle for alternative Banners #19729.

Relates to #19471

Testing done

N/A

Screenshots

QA steps

As a site admin

• [ ] Visit /admin/config/system/feature_toggle
• [ ] Confirm there is a toggle with the name FEATURE_BANNER_USE_ALTERNATIVE_BANNERS
• [ ] Validate the toggle is OFF

Select Team for PR review

• [ ] CMS Team
• [x] Public websites
• [ ] Facilities
• [ ] User support
• [ ] Accelerated Publishing

[github-actions[bot]]

Checking composer.lock changes...

[github-actions[bot]]

GitHub Workflows (.github/workflows/*.yml)

Have you...

• [ ] pinned all affected GitHub Actions at a specific commit by SHA?
• [ ] reviewed the source code of the action at the commit you are pinning?
• [ ] confirmed that no GitHub security measures are being bypassed?
• [ ] checked for any injection of user content into protected contexts?
• [ ] reviewed Security hardening for GitHub Actions?
• [ ] reviewed GitHub Workflows?