Support Behat 4 and Symfony 8

[stixx]

Behat 4 declares Controller::execute(): ?int and Extension::getConfigKey(): string. Four
implementations here lack those return types, which is a fatal error under Behat 4:

Declaration of DMarynicz\BehatParallelExtension\Cli\ParallelController::execute(...)
must be compatible with Behat\Testwork\Cli\Controller::execute(...): ?int

This adds them to ParallelController, RerunController, SigintController and Extension, and
widens the constraints to behat/behat: ^3.9 || ^4.0 plus || ^8.0 on the symfony components.

Adding a return type where the parent declares none is allowed, so Behat 3 keeps working. Constraints
are widened rather than moved — nothing is forced to upgrade.

Verified on both stacks, PHP 8.5

Behat 3.32.0, default resolution — the existing-user path:

phpstan	no errors
phpunit	78 tests, 189 assertions
behat	18 scenarios, 99 steps

symfony/process, serializer and expression-language resolve to v8.1.6 here. The other four stay
on 7.x because Behat 3 caps them.

Behat 4.0.0-alpha1, all seven symfony components on v8 — phpstan clean. Since the suite below can't
run on Behat 4 yet, parallel execution was verified in a separate consumer project:

serial:        1335ms   (3 scenarios, 3 passed)
--parallel 3:   903ms   (3/3, progress bar, real workers)

Not included: porting this project's own Behat suite

It still runs under Behat 3. Behat 4 removed both docblock annotations and YAML configuration, so
porting means converting every context to #[Given] attributes and all seven fixture behat.yml.dist
files to PHP — and those can't serve Behat 3 at the same time.

That makes it a decision about whether to drop Behat 3, which seemed yours to make rather than
something to slip into this PR. Happy to do that work separately if you'd like it, in either direction.

CI

Couldn't commit a workflow change (token lacks workflow scope). A Behat 4 leg would want its own
matrix entry, since the existing symfony key also raises console — which Behat 3 caps at ^7.0.