Add build test on PHP 8.0 & 8.1

Add `isTransactional` to `WallabagMigration` because PHP 8 behave differently with PDO transaction.
This is a workaround because we can't upgrade Doctrine Migration for now (upper versions have the fix).

- Build is now using Composer v2 (instead of v1)
- All actions have been updated to latest version
- Fix bug in PHP 8 were `$entry->getTags()` can't be properly used as a _traversable_ by `assertContains` during tests. Added a custom method `Entry::getTagsLabel()` which return a flatted tag array with only label
- Replace `assertNotRegExp` by `assertDoesNotMatchRegularExpression` because it was deprecated

tests/Wallabag/UserBundle/Controller/ManageControllerTest.php

@@ -65,7 +65,7 @@ class ManageControllerTest extends WallabagCoreTestCase
         $crawler = $client->followRedirect();
 
         // Check the user has been delete on the list
-        $this->assertNotRegExp('/Foo User/', $client->getResponse()->getContent());
+        $this->assertDoesNotMatchRegularExpression('/Foo User/', $client->getResponse()->getContent());
     }
 
     public function testDeleteDisabledForLoggedUser()