[FrameworkBundle] Simplified the MicroKernelTrait setup

[javiereguiluz]

Fixes #19975.

[yceruto]

Javier thanks for opening this PR so quickly :)

I was wondering if we could simplify the example even more by referencing the runtime autoload file instead and replace this code:

$kernel = new Kernel('dev', true);
$request = Request::createFromGlobals();
$response = $kernel->handle($request);
$response->send();
$kernel->terminate($request, $response);

with this other:

return static function (array $context) {
    return new Kernel($context['APP_ENV'], (bool) $context['APP_DEBUG']);
}

wdyt?

[javiereguiluz]

I like your proposal Yonel. I updated the PR with your changes. Thanks for the review.

[yceruto]

Don't forget to update the composer require section with symfony/runtime libs ;)

[yceruto]

In that section only symfony/framework-bundle is required actually, the other ones are transitive dependencies anyway, not sure we wanted to make them explicit though.

[javiereguiluz]

Good catch! I think that, generally speaking, we don't list transitive dependencies in the docs, so let's remove anything that is not strictly needed. Thanks!