Remove overly specific index-has-key constraint for link[@rel="proof-of-compliance"]

[aj-stein-gsa]

Committer Notes

Proposed fix that closes #2102.

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]
• Do all automated CI/CD checks pass?

By submitting a pull request, you are agreeing to provide this contribution under the CC0 1.0 Universal public domain dedication.

(For reviewers: The wiki has guidance on code review and overall issue review for completeness.)

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• ~Have you included examples of how to use your new feature(s)?
• Have you updated the OSCAL website and readme documentation affected by the changes you made? Changes to the OSCAL website can be made in the OSCAL-Pages and OSCAL_Reference repositories. N/A website documentation will be automatically updated from documentation generation pipeline.

[wandmagic]

there are multiple issues with this constraint:
it makes it so proof of compliance can only link to another component and that link must exactly match an existing UUID of a component.

this change will allow proof of compliance to be direct links as well as links to other components or the resources in the back-matter.

I would say that validated by should be removed as well, OR a #{uuid} prefixing with a hash for continuity with other self referential component links.

[aj-stein-gsa]

I am going to propose closing this PR in favor of the more narrowly scope constraint proposed #2107.