fix: Don't require submit buttons to have the 'filled' variant (#45247)

This PR also specifies the 'filled' variant for the question duplication
modal, since the previous default 'filled' variant for submit button was
added to make that specific button filled.

frontend/src/metabase/forms/components/FormSubmitButton/FormSubmitButton.tsx

@@ -44,7 +44,6 @@ export const FormSubmitButton = forwardRef(function FormSubmitButton(
       type="submit"
       color={submitColor}
       disabled={isDisabled}
-      variant="filled"
     >
       {submitLabel}
     </Button>

frontend/src/metabase/questions/components/CopyQuestionForm.tsx

@@ -88,7 +88,7 @@ export const CopyQuestionForm = ({
           {!!onCancel && (
             <Button type="button" onClick={onCancel}>{t`Cancel`}</Button>
           )}
-          <FormSubmitButton label={t`Duplicate`} />
+          <FormSubmitButton label={t`Duplicate`} variant="filled" />
         </FormFooter>
       </Form>
     </FormProvider>