Source freshness alerts ignore the description in meta #2396

Is your feature request related to a problem? Please describe.
Source freshness alerts always carry the default description from get_source_freshness_description() in the dbt package, and a description in the source's meta is ignored. Test alerts already use meta.description in place of their default. This came up in #1901, which was closed as stale.

We run freshness checks on a source loaded by Airbyte, and when one fails it nearly always means the connection has stopped syncing. The alert is the obvious place to say so.

Describe the solution you'd like
Source freshness alerts use meta.description (or meta.alerts_config.description) when it's set and keep the current default otherwise, the same as test alerts.

Describe alternatives you've considered
Swapping freshness for dbt_utils.recency tests, whose alerts do pick up meta.description. That works, but needs two tests per table to keep separate warn and error thresholds.

Additional context
Checked on 0.26.1 and master.

Would you be willing to contribute this feature?
Yes, #2395.