[5.3] Fix date format to use ISO 8601 in SchemaorgPrepareDateTrait

[LadySolveig]

Pull Request for Issue #45555 and #42196.

Summary of Changes

Fix date date format to use ISO 8601 (DateTime::ATOM) in in SchemaorgPrepareDateTrait.

Testing Instructions

1. Create or edit an article.
2. Enable structured data output using the Schema tab on the standard create article screen.
3. Choose any schema type that has a date field in the form (e. g. Article).
4. Use the built-in date picker to choose either a 'Date Created' or 'Date Modified' or both.
5. Validate the page using a structured data testing tool.

https://search.google.com/test/rich-results

• Apply the path via Patchtester or the testing package
• Repeat the steps above and check the result.

Actual result BEFORE applying this Pull Request

Expected result AFTER applying this Pull Request

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[ceford]

I have tested this item ✅ successfully on 2a86f07

With the patch applied I see the output date change from "dateModified": "2025-06-10", to "dateModified": "2025-06-10T00:00:00+00:00", and the Rich Results report passes (apart from flagging a Breadcrumbs problem).

But I wonder if this is the right solution. Should we store and output the time?

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/45581.}

[LadySolveig]

@ceford I think that's a bit of a question of taste. But I think the date and the correct format are particularly relevant for the search engines.
I am completely neutral here and don't really have an opinion.
Thank you for testing 💚