[date-time-picker] Do not mark as invalid when readonly

[web-padawan]

Description

In components where we use native <input> and call its checkValidity(), it returns true when the input is readOnly, as mentioned in #3750 (comment). We should mimic this behavior in vaadin-date-time-picker to not validate on blur when readonly.

Expected outcome

The component should not be invalid, like other components (vaadin-text-field, vaadin-combo-box).

Minimal reproducible example

<vaadin-date-time-picker required readonly></vaadin-date-time-picker>

Steps to reproduce

1. Add the snippet to the page
2. Press Tab to focus and then blur the date and time pickers

Environment

Vaadin version(s): Vaadin 22 - 23.2 alpha

Browsers

Issue is not browser related