[Round 2] Fix #9005: Allow singular tests to be documented in properties.yml

[aranke]

Resolves #9005

Problem

Take 2 of #10744 that was reverted in https://github.com/dbt-labs/dbt-core/pull/10790, accounting for scenario where customer has already used the data_tests YAML key.

Solution

Skip parsing and warn the user that the data_tests key is now used for data test documentation.

Checklist

• [x] I have read the contributing guide and understand what's expected of me.
• [x] I have run this code in development, and it appears to resolve the stated issue.
• [x] This PR includes tests, or tests are not required or relevant for this PR.
• [x] This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• [x] This PR includes type annotations for new and modified functions.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.76471% with 9 lines in your changes missing coverage. Please review.

Project coverage is 89.17%. Comparing base (2ff3f20) to head (cdf5c26). Report is 5 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #10792 +/- ## ========================================== + Coverage 89.11% 89.17% +0.06% ========================================== Files 183 183 Lines 23288 23456 +168 ========================================== + Hits 20752 20916 +164 - Misses 2536 2540 +4 ``` | [Flag](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10792/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10792/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `86.41% <86.76%> (+0.09%)` | :arrow_up: | | [unit](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10792/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `62.09% <41.17%> (-0.11%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs#carryforward-flags-in-the-pull-request-comment) to find out more. | [Components](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10792/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | Coverage Δ | | |---|---|---| | [Unit Tests](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10792/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `62.09% <41.17%> (-0.11%)` | :arrow_down: | | [Integration Tests](https://app.codecov.io/gh/dbt-labs/dbt-core/pull/10792/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dbt-labs) | `86.41% <86.76%> (+0.09%)` | :arrow_up: |