[2.3] Fix libxml_use_internal_errors and libxml_disable_entity_loader usage

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9731
| License       | MIT

This should fix #9731 that seems to be triggered when `libxml_disable_entity_loader` has been called with `true` (see https://bugs.php.net/bug.php?id=62577)
As `libxml_disable_entity_loader` is non thread safe (see https://bugs.php.net/bug.php?id=64938) and as we have some calls that might leave the setting to `true`, I think the bug should be fixed.

I've checked the use of both `libxml_use_internal_errors` and `libxml_disable_entity_loader` among symfony code.

You can see I prefered to skip DomDocument::loadXML warnings using the `@` instead of using `LIBXML_NOERROR | LIBXML_NO_WARNING` because we can log these errors whereas libxml errors would be furtives.