Fix custom attributes on lists with one item

[rmccar]

What is the context of this PR?

Fixes: #3332

This PR updates the list macro to set custom attributes on the correct elements when you have a list with only one item that is a link.

Added comment as reminder for next breaking change to list component

How to review this PR

• Test the correct attributes are set using these configurations:

{% from "components/list/_macro.njk" import onsList %}
{{
    onsList({
        "element": "ol",
        "attributes": {
            "data-qa": "error-list"
        },
        "itemsList": [
            {
                "url": '#0',
                "text": 'Who we are',
                "attributes": {
                    "data-qa": "error-list-1"
                }
            }
        ]
    })
}}

{% from "components/list/_macro.njk" import onsList %}
{{
    onsList({
        "element": "ol",
        "attributes": {
            "data-qa": "error-list"
        },
        "itemsList": [
            {
                "url": '#0',
                "text": 'Who we are'
            }
        ]
    })
}}

• Test other configurations with more than one item in the list with and without setting the url param on the items

Checklist

This needs to be completed by the person raising the PR.

• I have selected the correct Assignee
• I have linked the correct Issue

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