Ubercart storing `uc_store.settings.date_format_uc_store` but it should be stored in `system.date.formats`

[herbdool]

In Drupal 7 to Backdrop all variables with the format of date_format_NAME were converted to be stored in system.date like this:

     "formats": {
        "long": {
            "label": "Long",
            "pattern": "l, j F, Y - g:ia",
            "module": "system"
        },

This way the format will appear in the "Date and Time Formats" settings UI. I'm guessing that right now this config isn't exposed anywhere.

[herbdool]

@argiepiano @bugfolder I've got a PR to migrate this variable to system.date. I updated the usage in Ubercart, but I left the old variable there. It's used in uc_bank_transfer which would need the old one for now. When I ported uc_coupon I had already put in a helper to look in both locations.

[bugfolder]

Custom modules built on Ubercart might also use the old format location (I have one on a site that uses it), so leaving the old one in its config is definitely a good thing to do. However, that one will no longer be editable, so owners of custom modules need to be notified that they'll need to update their module. The best way I can think of to do that is to return a string from the update hook, which will be displayed when the user upgrades, so that site developers will at least see this when they run the upgrade. (Of course, we should also include that information in the upgrade notes.)

So perhaps add something like: "This upgrade replaces the date format uc_store.settings.date_format_uc_store with system.dates.formats.uc_store.pattern. Custom modules that access the date format should be modified to use the new location."