[Button] `disabled` attribute is not allowed on element `a`

[kaelig]

Issue summary

A disabled button renders invalid markup when passed a URL prop:

<Button disabled={true} url="https://foo.com">Add product</Button>

Expected behavior

HTML is valid, without a disabled attribute on the anchor element:

<a class="Polaris-Button Polaris-Button--disabled" href="https://foo.com" data-polaris-unstyled="true"><span class="Polaris-Button__Content"><span>Add product</span></span></a>

Actual behavior

HTML is invalid (Error: Attribute disabled not allowed on element a at this point):

<a class="Polaris-Button Polaris-Button--disabled" disabled="" href="https://foo.com" data-polaris-unstyled="true"><span class="Polaris-Button__Content"><span>Add product</span></span></a>

diff:

- <a class="Polaris-Button Polaris-Button--disabled" disabled="" href="https://foo.com"
+ <a class="Polaris-Button Polaris-Button--disabled" href="https://foo.com"

Steps to reproduce the problem

1. Go to https://codesandbox.io/s/nrz4zwlnqj?module=App.js
2. Look at the rendered markup
3. Validate markup against https://validator.w3.org/nu/#textarea

Specifications

• Polaris version number: 3.3.0