minor #36976 [Contracts/Deprecation] fix composer.json for PHP 8 (xepozz)

This PR was merged into the 5.1 branch.

Discussion
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[Contracts/Deprecation] fix composer.json for PHP 8

| Q             | A
| ------------- | ---
| Branch?       | 5.1
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

Spotted by @xepozz in https://github.com/symfony/deprecation-contracts/pull/2

Commits
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5db0a8bf4b [Contracts/Deprecation] fix composer.json for PHP 8

src/Symfony/Contracts/Deprecation/composer.json

@@ -15,7 +15,7 @@
         }
     ],
     "require": {
-        "php": "^7.1"
+        "php": ">=7.1"
     },
     "autoload": {
         "files": [