Allow Trailing slash on void elements has no effect and interacts badly with unquoted attribute values on html validation

When upgrading validator to 25.5.12 (XWIKI-23197), a new warning error was introduced.

Trailing slash on void elements has no effect and interacts badly with unquoted attribute values.

This warning error is raised whenever a self-closing element is closed with a /
For instance, <img src='img.png' /> is raising an error, but <img src='img.png'> is ok.

The issue is widespread and impacting many elements: img, meta, input, link, etc.
Removing all the errors at once is a huge task and we might need to find a way to fix this incrementally.