Default value for metadata.dbt_version are confusing.

**Current Behaviour**
The default value for dbt_version is the version of dbt that's currently running. i.e the code is `dbt_version: str = __version__`. However, in the docs the default version is 1.6.0a1, since this was the dbt version running when the manifest was generated. This confused our parters who thought the default represented the first dbt version to use this schema.

**Expected Behaviour**
The default should say something along the line of "the dbt version that's currently running i.e 1.6.0a1"