[Configuration] Load single composer-based Symfony set in withComposerBased()

[TomasVotruba]

Same treatment as PHPUnit in #8255: withComposerBased(symfony: true) now loads the single composer-based Symfony set instead of the whole SetGroup::SYMFONY group.

Every rule in that set declares the exact Symfony package + version its target API was added in, so there is no need to load all version sets and let them overlap.

 if ($phpunit) {
     // single set, as every rule inside is bound to the installed PHPUnit version on its own
     $this->sets[] = PHPUnitSetList::COMPOSER_BASED;
 }
+
+if ($symfony) {
+    // single set, as every rule inside is bound to the installed Symfony package version on its own
+    $this->sets[] = SymfonySetList::COMPOSER_BASED;
+}

Blocked until SymfonySetList::COMPOSER_BASED lands in rector-symfony dev-main - CI is red until then.