Update symfony/string requirement from ^6.0 to ^7.1

Updates the requirements on [symfony/string](https://github.com/symfony/string) to permit the latest version.
- [Release notes](https://github.com/symfony/string/releases)
- [Changelog](https://github.com/symfony/string/blob/7.1/CHANGELOG.md)
- [Commits](symfony/string@v6.0.0...v7.1.1)

---
updated-dependencies:
- dependency-name: symfony/string
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

composer.json

@@ -33,7 +33,7 @@
         "symfony/finder": "^6.0",
         "symfony/http-foundation": "^6.0",
         "symfony/mime": "^6.0",
-        "symfony/string": "^6.0"
+        "symfony/string": "^7.1"
     },
     "require-dev": {
         "dereuromark/composer-prefer-lowest": "^0.1.10",