Support v4 or v5 of symfony/yaml to address some composer conflicts

Author: jasonvarga

composer.json

@@ -21,7 +21,7 @@
         "spatie/blink": "^1.1.2",
         "statamic/stringy": "^3.1",
         "symfony/var-exporter": "^4.3",
-        "symfony/yaml": "^4.1@dev",
+        "symfony/yaml": "^4.1 || ^5.1",
         "twistor/flysystem-guzzle": "^6.0",
         "ueberdosis/html-to-prosemirror": "^0.7.0",
         "ueberdosis/prosemirror-to-html": "^0.10.0",