The "strict" option has been dropped in loaders

> "strict" has been deprecated in 2.8, removed from 3.0, but loaders still read them
> (note that the XSD still mentions it for BC at XML level, but it has no effect.)

https://github.com/symfony/symfony/pull/20938

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We probably don't mention "strict" in the framework docs ... but we should double check that components don't mention it either.
