use lower case for Croatian Kuna symbol

[rrennick]

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:

Use lower case for Croatian Kuna symbol. In addition to screenshot on issue, ref https://en.wikipedia.org/wiki/Croatian_kuna

Closes #21931 .

How to test the changes in this Pull Request:

1. Go to the WC General Settings screen.
2. Open the Currency dropdown & scroll to Croatia.

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 successfully ran tests with your changes locally?

Changelog entry

Use lower case for Croatian Kuna symbol.

[claudiulodro]

Looks good. Thanks!