Update source.md (#4884)

## What are you changing in this pull request and why?
That note is no longer relevant since all versions before 1.0 are
deprecated.

## 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.

website/docs/reference/commands/source.md

@@ -6,9 +6,7 @@ id: "source"
 
 The `dbt source` command provides subcommands that are useful when working with source data. This command provides one subcommand, `dbt source freshness`.
 
-:::info
-If you're using an older version of dbt Core (before v0.21), the old name of the `freshness` subcommand was `snapshot-freshness`. (It has nothing to do with [snapshots](/docs/build/snapshots), which is why we renamed it.) Each time you see the command below, you'll need to specify it as `dbt source snapshot-freshness` instead of `dbt source freshness`.
-:::
+
 
 ### dbt source freshness