Custom D7 date formats not migrated to D9

This is a bugsmash daily triage target.

@DamienMcKenna, thanks for making the patch!

This needs confirmation that the changes to the D7 fixture are correct and that the fixture can still be used on a D7 site via the UI. That is, that is it still a working database. This also needs a source plugin test and a migration test.

Thanks for the patch @DamienMcKenna

Looking at it covers date formats (i.e. l, F j, Y - H:i), but no date types. (e.g. Long / date_format_long). I think date formats and date types are not two separate configuration entities in Drupal 11. The date type can be mapped to the id/label of the DateFormat configuration entity, and the date format mapped to the pattern. This would require querying the date_format_type and date_formats respectively. Thoughts?