Replace VALIDATE core action with VALIDATE_DECLARATION custom action

[cibelius]

Description

Clearly describe what has been changed. Include relevant context or background.
Explain how the issue was fixed (if applicable) and the root cause.

Link this pull request to the GitHub issue (and optionally name the branch ocrvs-<issue #>)

Checklist

• I have linked the correct Github issue under "Development"
• I have tested the changes locally, and written appropriate tests
• I have tested beyond the happy path (e.g. edge cases, failure paths)
• I have updated the changelog with this change (if applicable)
• I have updated the GitHub issue status accordingly

[github-actions]

Oops! Looks like you forgot to update the changelog. When updating CHANGELOG.md, please consider the following:

• Changelog is read by country implementors who might not always be familiar with all technical details of OpenCRVS. Keep language high-level, user friendly and avoid technical references to internals.
• Answer "What's new?", "Why was the change made?" and "Why should I care?" for each change.
• If it's a breaking change, include a migration guide answering "What do I need to do to upgrade?".