[travis] use PHP 8.0 to patch return types and run deps=low

Author: nicolas-grekas

composer.json

@@ -35,7 +35,7 @@
         "symfony/property-access": "^3.4|^4.0|^5.0",
         "symfony/property-info": "^3.4|^4.0|^5.0",
         "symfony/translation": "^4.2",
-        "doctrine/annotations": "~1.7",
+        "doctrine/annotations": "^1.10.4",
         "doctrine/cache": "~1.0",
         "egulias/email-validator": "^2.1.10"
     },