[CT-2701] Snapshot dbt_updated_at not updated

### Is this a new bug in dbt-core?

- [X] I believe this is a new bug in dbt-core
- [X] I have searched the existing issues, and I could not find an existing issue for this bug

### Current Behavior

This was brought up by a customer and I am kinda surprised I have never heard this before.

[Our docs show](https://docs.getdbt.com/docs/build/snapshots)

two fields in the existing row should be updated when the `updated_at` timestamp is changed signifying a change of some value, the `dbt_valid_to` and the `dbt_updated_at`


Looks like only one update is happening and the `dbt_updated_at` does not get updated.


### Expected Behavior

According to the docs, the older row should have two fields updated:
`dbt_valid_to`
`dbt_updated_at`

### Steps To Reproduce

- create snapshot with timestamp check
- add record `1 as id, '2023-01-01' as updated_at`
- run snapshot
- modify record `1 as id, '2023-01-02' as updated_at`
- run snapshot
- check the records in the output and see if `dbt_updated_at` has been modified in the original row

### Relevant log output

_No response_

### Environment

```markdown
- OS:
- Python:
- dbt: tested 1.1 and 1.4
```

### Which database adapter are you using with dbt?

_No response_

### Additional Context

_No response_