fix(image-with-caption): adjust image button background color (#10279)

### Related Ticket(s)

#10250
https://jsw.ibm.com/browse/ADCMS-3105

### Description

Sets the background style to transparent for the `<button>` element used in the shadow DOM of the `<dds-image>` and `<dds-image-with-caption>` components, such that when used with a transparent image (SVG, transparent gif/png), the browser's default button grey does not show through.

Note that the original issue was reported for `<dds-image-with-caption>`, but the issue equally effects `<dds-image>` component as well. Note also that the `<dds-image-with-caption>` button was recently deprecated, it's features merged with `<dds-image>`. Use of `<dds-image-with-caption>` should be replaced by `<dds-image>` (see #8839). The deprecation is documented in the [Storybook README for the Image component](https://carbon-design-system.github.io/carbon-for-ibm-dotcom/canary/web-components/?path=/docs/components-image--default#image-with-caption).

### Testing Instructions

- [ ] Open Storybook to the Image > Default component. 
- [ ] For the "Default image" Storybook knob, choose "SVG". A transparent SVG is used as the image for the component. 
- [ ] Select Lightbox as well from the Storybook knobs
- [ ] Note the browser default button color does not show through.

### Changelog

**Changed**

- Added `background-color: transparent` to the button styles for the `<dds-image>` component when in lightbox mode.

packages/styles/scss/components/image-with-caption/image-with-caption.scss

@@ -1,5 +1,5 @@
 /**
- * Copyright IBM Corp. 2016, 2022
+ * Copyright IBM Corp. 2016, 2023
  *
  * This source code is licensed under the Apache-2.0 license found in the
  * LICENSE file in the root directory of this source tree.
@@ -32,6 +32,7 @@
     position: relative;
     pointer-events: none;
     border: none;
+    background-color: transparent;
 
     &:hover {
       // only have lightbox functionality on bigger breakpoints

packages/web-components/src/components/content-block-simple/__stories__/content-block-simple.stories.ts

@@ -63,7 +63,6 @@ const copy = `Lorem ipsum *dolor* sit amet, consectetur adipiscing elit. Aenean
         - list item 2a.a
 `;
 
-// TODO: Replace with <dds-image-with-caption>
 const image = html`
   <dds-image slot="media" alt="Image alt text" default-src="${imgLg16x9}">
     <dds-image-item media="(min-width: 672px)" srcset="${imgLg16x9}">