Remove custom today XPath function

[jkuester]

Description

This PR removes our custom today XPath function (allowing us to inherit the default functionality from the OpenRosa today function. The difference is that our custom implementation of today would include the current time value while OpenRosa's default implementation (which matches JavaRosa) is that today only returns today's date at midnight in the current timezone.

Form changes:

In places where we were using today, If we do not actually care about the time value, I just let it be. If it seemed like the time value might be relevant, I switched to use now. When today was being wrapped with a floor call, I just removed the floor since it should no longer be necessary (see the discussion on the issue for more details about this). Finally, I also tried to remove unnecessary calls to decimal-date-time as I went. We were using these a lot when comparing date values, but this is unnecessary. (Enketo supports comparing date values with <>=. They do not have to be converted to numbers.)

Closes #7731

Code review checklist

• Readable: Concise, well named, follows the style guide, documented if necessary.
• Tested: Unit and/or e2e where appropriate
• Backwards compatible: Works with existing data and configuration or includes a migration. Any breaking changes documented in the release notes.

License

The software is provided under AGPL-3.0. Contributions to this project are accepted under the same license.

[njogz]

Great job on this! Is there another issue where we are keeping track of Enketo behaviour changes that app builders should be aware of and how to overcome them?

[jkuester]

@njogz good call. I don't think there is an issue, but I will be sure to include this in the documentation changes we are adding in medic/cht-docs#802