Missing changelog links for 1.7

### Problem

While the changelogs for 1.7 _do_ exist for our adapters, we're missing links to them in the main changelog:

It's not a current problem within the [dbt-core changelog](https://github.com/dbt-labs/dbt-core/blob/main/CHANGELOG.md#previous-releases).

### Proposed solution

```[tasklist]
### My tasks
- [ ] https://github.com/dbt-labs/dbt-bigquery/blob/main/CHANGELOG.md
- [ ] https://github.com/dbt-labs/dbt-redshift/blob/main/CHANGELOG.md
- [ ] https://github.com/dbt-labs/dbt-snowflake/blob/main/CHANGELOG.md
- [ ] https://github.com/dbt-labs/dbt-spark/blob/main/CHANGELOG.md
```

### Acceptance criteria

- Under the "Previous Releases" heading, there is an item in the list for "1.7"
- It hyperlinks to a file like [this](https://github.com/dbt-labs/dbt-bigquery/blob/1.7.latest/CHANGELOG.md)