Fix Bogus DNSSEC Validation Domain in unbound.md

[HeliusMagnum]

Thank you for your contribution to the Pi-hole Community!

Please read the comments below to help us consider your Pull Request.

We are all volunteers and completing the process outlined will help us review your commits quicker.

Please make sure you

1. Base your code and PRs against the repositories developmental branch.
2. Sign Off all commits as we enforce the DCO for all contributions
3. Sign all your commits as they must have verified signatures
4. File a pull request for any change that requires changes to our documentation at our documentation repo

---

What does this PR aim to accomplish?:

Fix the issue with fail01.dnssec.works returning NOERROR

How does this PR accomplish the above?:

Changes the test domain for a bogus DNSSEC validation to bogus.nlnetlabs.nl which returns SERVERFAIL

Changes the test domain for a successful DNSSEC validation to cloudflare.com to avoid any future misconfiguration with the dnssec.works domains, specifying the +ad flag.

resolves #1251

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)
6. I have checked that another pull request for this purpose does not exist.
7. I have considered, and confirmed that this submission will be valuable to others.
8. I accept that this submission may not be used, and the pull request closed at the will of the maintainer.
9. I give this submission freely, and claim no ownership to its content.

---

• I have read the above and my PR is ready for review. Check this box to confirm

[netlify]

✅ Deploy Preview for pihole-docs ready!

Name	Link
🔨 Latest commit	419117c
🔍 Latest deploy log	https://app.netlify.com/projects/pihole-docs/deploys/684a663ded87cc0008fcf54d
😎 Deploy Preview	https://deploy-preview-1252--pihole-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.