minor symfony#79 Allow installation on php 8 (derrabus)

This PR was merged into the 2.0-dev branch.

Discussion
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Allow installation on php 8

Commits
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9243f93 Allow installation on php 8.

Author: nicolas-grekas

composer.json

@@ -16,7 +16,7 @@
         }
     ],
     "require": {
-        "php": "^7.1",
+        "php": ">=7.1",
         "psr/http-message": "^1.0",
         "symfony/http-foundation": "^4.4 || ^5.0"
     },