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

Author: nicolas-grekas

composer.json

@@ -39,11 +39,11 @@
         "symfony/validator": "^4.4.2|^5.0.2",
         "symfony/var-dumper": "^3.4|^4.0|^5.0",
         "symfony/translation": "^3.4|^4.0|^5.0",
-        "doctrine/annotations": "~1.7",
+        "doctrine/annotations": "^1.10.4",
         "doctrine/cache": "~1.6",
         "doctrine/collections": "~1.0",
         "doctrine/data-fixtures": "^1.1",
-        "doctrine/dbal": "~2.4|^3.0",
+        "doctrine/dbal": "^2.6|^3.0",
         "doctrine/orm": "^2.6.3"
     },
     "conflict": {