ci: align the whole symfony stack on the 8.1 edge job

[soyuka]

Makes PHPUnit (PHP 8.5) (Symfony 8.1) report on Symfony 8.1 instead of on an unsupported dependency mix.

Problem

The job sets minimum-stability: dev but pins only two packages:

composer require --dev --no-update "symfony/framework-bundle:8.1.x-dev" "symfony/json-streamer:8.1.x-dev"

Everything else then floats to the next minor's dev branch. The actual resolution is:

package	resolved
symfony/framework-bundle	8.1.x-dev (pinned)
symfony/json-streamer	8.1.x-dev (pinned)
symfony/serializer	8.2.x-dev
symfony/property-info	8.2.x-dev
symfony/type-info	8.2.x-dev

Mixing Symfony minor dev branches is not supported, and here it silently breaks property discovery — nested objects serialize as {} and interface-backed DTO properties disappear:

JsonLd\InterfaceDtoOutputTest::testCollectionExposesOnlyInterfaceProperties
Failed asserting that an array has the key 'name'.

Serializer\ConstructorDeserializationTest::testPostHydratesObjectViaConstructor
-      '@type' => 'DummyObjectWithoutConstructor',
-      'foo' => 'bar',

Evidence

Reproduced locally with the job's own recipe, then varied only the resolution:

resolution	result
framework-bundle 8.1.x-dev + serializer/property-info/type-info 8.2.x-dev (current job)	both tests fail
everything 8.2.x-dev	both pass
everything 8.1.x-dev	both pass

So the failures come from the mix, not from API Platform and not from any Symfony version on its own. The Symfony dev job — which sets minimum-stability: dev without pins, so everything lands on 8.2.x-dev consistently — does not report these two failures, which matches.

Fix

Pin the serializer stack alongside framework-bundle so the job resolves a coherent 8.1 line.

The job stays continue-on-error: true; this is about the signal being meaningful rather than about turning it into a gate. Without this it reports failures that no user can hit, which is worse than reporting nothing.