[FIX] website: give correct height to image gallery on smaller screens

Prior to this commit, the height of the Image Gallery snippet was set to
auto on screens smaller than 400px.

This ensures that the snippet looks the best on phones.

However, when using Bootstrap, what defines a smaller screen is not
any screen below 400px, but any screen below 768px.
This meant that the Image Gallery snippet did not look the same on an
iPhone 11 Pro max as it would on an iPhone 11 for example.

This commit fixes that by using the built-in mixin that relies on
Bootstrap's breakpoints (in this case SM).

Steps to reproduce:
- Go in edit mode and drop an "Image Gallery" snippet
- Open the dev tools and choose "iPhone 11" or 375x812
- The image gallery does not have white bands
- Change the resolution to "iPhone 11 Pro Max" or 414x896
- The image gallery has white bands / has a different layout

opw-2995100
task-2997119

closes odoo/odoo#114297

X-original-commit: c36607dc
Signed-off-by: Vray Benjamin (bvr) <bvr@odoo.com>