Make coupon code label accessible

[IanDelMar]

All Submissions:

• Have you followed the WooCommerce Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Currently the coupon code label in the cart template is hidden by display: none. The label is removed from the visual flow of the page and is ignored by screen readers. To make the label accessible this PR removes the display: none and adds the screen-reader-text class to hide the label.

• This PR is a very minor change/addition and does not require testing instructions (if checked you can ignore/remove the next section).

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you created a changelog file for each project being changed, ie pnpm --filter=<project> changelog add?

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.

[barryhughes]

Thank you, looks good!

Let's wait on the suite of automated checks passing, then we can move ahead and merge.

[barryhughes]

Re-running a couple of failed checks (probably flaky tests).