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/CoreBundle/Controller/EntryControllerTest.php

@@ -342,7 +342,7 @@ class EntryControllerTest extends WallabagCoreTestCase
         $entry = $em
             ->getRepository('WallabagCoreBundle:Entry')
             ->findOneByUrl($url);
-        $tags = $entry->getTags();
+        $tags = $entry->getTagsLabel();
 
         $this->assertCount(2, $tags);
         $this->assertContains('wallabag', $tags);
@@ -372,7 +372,7 @@ class EntryControllerTest extends WallabagCoreTestCase
             ->getRepository('WallabagCoreBundle:Entry')
             ->findOneByUrl($url);
 
-        $tags = $entry->getTags();
+        $tags = $entry->getTagsLabel();
 
         $this->assertCount(2, $tags);
         $this->assertContains('wallabag', $tags);