BUG: via.placeholder.com does not exist anymore

### GrapesJS version

- [x] I confirm to use the latest version of GrapesJS

### GrapesJS MJML version

- [x] I confirm to use the latest version of GrapesJS MJML

### What browser are you using?

latest version

### Reproducible demo link

https://grapesjs.com/demo-mjml.html

### Describe the bug

**How to reproduce the bug?**
1. Add a new image via drag and drop
2. the browser cant resolve https//via.placeholder.com

**What is the expected behavior?**
We should see a placeholder image with the dimensions 350x250

**What is the current behavior?**
hundreds of error messages in the console and no placeholder image

**Solution**
replace via.placeholder.com with https://placehold.co. The syntax for dimensions and color is the same meaning "/350x250/78c5d6/fff" is still valid

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct