Expose a setting to opt-in for the XSD validation for mapping

[stof]

Closes #1647

[ostrolucky]

This means we can remove issue from https://github.com/doctrine/DoctrineBundle/blob/2.9.x/Tests/baseline-ignore now, right?

[stof]

@ostrolucky indeed. I removed it now (removing the baseline file entirely as it is empty) by fixing the remaining test triggering the error.

[stof]

@ostrolucky @dmaicher I suggest that once this PR is merged, we trigger the 2.10.0 release. This would allow shipping this code to projects upgrading to Symfony 6.3 to avoid getting new reports about deprecations. I see no reason to delay the release (we have nothing else in-progress and that's the last deprecation was can solve in the bundle itself as the remaining one is in the ORM code)