Ban CREATE SOURCE for non-backfillable CDC source

[xxchan]

And the original question: Can we CREATE SOURCE for citus-cdc? If not, where is it rejected?

We can't. But it seems we didn't ban it in code.

   // gated the feature with a session variable
    let create_cdc_source_job = if with_properties.is_cdc_connector() {
        CdcTableType::from_properties(&with_properties).can_backfill()
    } else {
        false
    };

Originally posted by @StrikeW in #15635 (comment)

[github-actions]

This issue has been open for 60 days with no activity. Could you please update the status? Feel free to continue discussion or close as not planned.