Restore xml-common version

Author: tvdijen

composer.json

@@ -15,7 +15,7 @@
         "ext-pcre": "*",
 
         "simplesamlphp/assert": "~2.0",
-        "simplesamlphp/xml-common": "~2.0"
+        "simplesamlphp/xml-common": "dev-feature/dom-migration-php84"
     },
     "require-dev": {
         "simplesamlphp/simplesamlphp-test-framework": "~1.11"