fix(ally): Ensure buttons have discernible text

[larowlan]

We're using the nsw-design system in a storybook setup.
In that setup we have storybook's test runner using playwright to do axe testing of a11y.
The search input when there's an icon is failing with a critical error 'Ensure buttons have discernible text'. This is because the icon is the only markup inside the button, and this is set to aria-hidden=false.
This change creates a minor visual difference in it's current form.

[laurenhitchon]

Hi @larowlan,

Thanks again for your contribution.

Adding a visually hidden with the sr-only class to provide discernible text for screen readers is a great solution to address the critical error 'Ensure buttons have discernible text' flagged by the axe tests. While we understand that this change might introduce a minor visual difference, the improvement in accessibility is highly valuable.

We'll verify and test that this update does not impact the button's functionality or introduce any unintended visual discrepancies across different themes and then merge this PR early next week if there are no issues.