Rename config key dns.domain > dns.domain.name

[yubiuser]

What does this PR aim to accomplish?:

Accompanies pi-hole/FTL#2531

Rename config key dns.domain > dns.domain.name

---

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

[DL6ER]

HOLD merging until pi-hole/FTL#2531 is merged, too.

[yubiuser]

FTL PR has been merged.