DEVDOCS-6496 - Clarify date format for modifiers in documentation

[bc-terra]

Updated documentation for date modifier to specify ISO-8601 ATOM format.

DEVDOCS-6496

What changed?

• When sending a PUT or POST request to create/update product modifier options, the full ISO-8601 ATOM format is required.
• Currently, if the wrong format is provided, such as “2005-08-15”, validation should fail, but it doesn’t.

Release notes draft

• We've updated the validation workflow for creating product modifiers to require ISO-8601 ATOM format, which includes date, time, and timezone. Failing to provide dates in ISO-8601 ATOM format will now result in a validation error.

Anything else?

ping { @bigcommerce/dev-docs-team }

[chris-nowicki]

LGTM 🚀