Update documentation on yml version defaults (#5672)

## What are you changing in this pull request and why?
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The [config-version
documentation](https://docs.getdbt.com/reference/project-configs/config-version)
is not clear about what the default value is. It states that the
parameter is optional and that the old version, version 1, is not in
use. This implies that the default version should be 2, but the default
is explicitly stated here as version 1.
```md
## Default
Without this configuration, dbt will assume your dbt_project.yml uses the version 1 syntax, which was deprecated in dbt v0.19.0.
```

As per PR dbt-labs/dbt-core#7060, the default
version is in-fact version 2, so this documentation needs to be updated.

## Checklist
- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.
- [x] For [docs
versioning](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#about-versioning),
review how to [version a whole
page](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
and [version a block of
content](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#versioning-blocks-of-content).
- [x] Add a checklist item for anything that needs to happen before this
PR is merged, such as "needs technical review" or "change base branch."

---------

Co-authored-by: Mirna Wong <[email protected]>

website/docs/reference/project-configs/config-version.md

@@ -23,4 +23,4 @@ Specify your `dbt_project.yml` as using the v2 structure.
 <VersionBlock firstVersion="1.5"> This configuration is optional. </VersionBlock>
 
 ## Default
-Without this configuration, dbt will assume your `dbt_project.yml` uses the version 1 syntax, which was deprecated in dbt v0.19.0.
+Without this configuration, dbt will assume your `dbt_project.yml` uses the version 2 syntax. Version 1 was deprecated in dbt v0.19.0.