fix: Wrong class for the clear button slot

[Tsuyoshi84]

Describe your changes

Because the old class, dp__clear_icon, was still applied to the parent element of the clear button slot, the clear button was misplaced in the field.

This PR fixes the class so the button is positioned correctly.

Here are the results:

Before fix

After fix

The code

```vue ```

Issue ticket number and link

https://github.com/Vuepic/vue-datepicker/issues/945

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [x] I have ensured that unit tests pass without errors
• [x] If it is a new feature, I have added a new unit test