Fix Metaschema validation errors

[DimitriZhurkin]

Committer Notes

In the has-security-sensitivity-level and has-security-impact-level constraints, move the <message> elements below the help-url props.

All Submissions:

• Have you selected the correct base branch per Contributing guidance?
• Have you set "Allow edits and access to secrets by maintainers"?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you squashed any non-relevant commits and commit messages? [instructions]
• Have you added an explanation of what your changes do and why you'd like us to include them?
• If applicable, have all FedRAMP Documents Related to OSCAL Adoption affected by the changes in this issue have been updated.?
• If applicable, does this PR reference the issue it addresses and explain how it addresses the issue?

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[Gabeblis]

@DimitriZhurkin Good catch. I included this in #856, but you definitely did it the right way sir 🫡

[Gabeblis]

Good fix