Related Posts: thumbnails are not displayed correctly (aspect ratio is not preserved) if block theme is used

### Impacted plugin

Jetpack

### Steps to Reproduce

1. Activate Twenty Twenty-Three theme
2. Activate the Related Posts feature under Jetpack > Settings > Traffic
3. Create a few posts (4) with featured images
4. Add **Related Posts** blog to the post, make sure to check **Display thumbnails** in the Related Posts settings
5. Visit the post in the front-end view, make sure that your viewport size is larger than 640px

### A clear and concise description of what you expected to happen.

Thumbnails are displayed using their correct aspect ratios.

### What actually happened

You will notice that the aspect ratio of thumbnails is not preserved.
When you check the styles applied to the image, you will notice that the following `width` rule is applied:

```
.jp-related-posts-i2__post-img-link img {
width: 100%;
}
```

**but there is no rule for height** (like `height:auto`). In that case, the browser uses the `height` attribute from the `img` markup (200px) instead.

### Browser

Google Chrome/Chromium, Mozilla Firefox, Microsoft Edge, Apple Safari

### Other information

The issue does not occur in (probably most) classic themes since they add general rules for `img`, for instance Twenty Twenty-One adds this rule:

```
img {
height: auto;
max-width: 100%;
vertical-align: middle;
}
```

### Platform (Simple, Atomic, or both?)

Self-hosted

### Reproducibility

Consistent

### Severity

Some (< 50%)

### Available workarounds?

Yes, easy to implement

### Workaround details

Go to /wp-admin/customize.php > Additional CSS and add

```
img {
height: auto;
max-width: 100%;
}
```

Reproduce the issue with Jetpack Related Posts enabled on Twenty Twenty-Three, using four posts with featured images and a viewport wider than 640px. Inspect the `.jp-related-posts-i2__post-img-link img` styles and confirm the fix preserves thumbnail aspect ratios in the front end without relying on Additional CSS.

Written by the indexing model from the issue text.