[PHP-Symfony] Fix Symfony warning on OpenAPIServerBundle

[gturri]

This fixes the issue described on #15959 ; it: it fixes the Symfony warning:

User Deprecated: Method "Symfony\Component\HttpKernel\Bundle\Bundle::build()" might add "void" as a native return type declaration in the future. Do the same in child class "OpenAPI\Server\OpenAPIServerBundle" now to avoid errors or add an explicit @return annotation to suppress this message

I tested this PR by setting up a Symfony app with the bundle autogenerated with this patch, and observing that the warning goes away, and that the queries made against the symfony app are correctly handled.

It's PHP, so here is a ping: @jebentier, @dkarlovi, @mandrean, @jfastnacht, @ybelenko, @renepardon

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