Update the date input to use text inputs with a numeric input mode

[36degrees]

What

Update the date input so that each of the inputs is of type="text" with inputmode="numeric", rather than using type="number".

For now, we're going to keep the pattern attribute as it ensures the right keyboard is displayed for iOS < 12.2+ – we can review this when we stop seeing much traffic from these older versions.

Why

For context, see the results of the investigation in #1449

Who needs to know about this

Developers

Done when

• Inputs within the date input component use type="text"
• Inputs within the date input component use inputmode="numeric"
• Tests for the above
• Tested in the browsers / assistive technologies that we support

We should then:

• Verify that Dragon Accessibility issue with DOB field #1101 is fixed and close it
• Close Add Dragon bug to date input guidance govuk-design-system#782
• Verify that NVDA not assigning labels properly in date input component #1408 is fixed and close it
• Remove "Attribute pattern is only allowed when the input type is email, password, search, tel, text, or url" from our list of known errors / warnings as we are now be using input type="text" which won't trigger validation warnings.

[36degrees]

@htmlandbacon has started working on this in #1599 – I've asked him if he's happy to make some changes to the PR, and we can pick this up next week.