Update symfony/flex requirement from ^1.0 to ^2.4

Updates the requirements on [symfony/flex](https://github.com/symfony/flex) to permit the latest version.
- [Release notes](https://github.com/symfony/flex/releases)
- [Commits](symfony/flex@v1.0.0...v2.4.5)

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

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

Author: dependabot[bot]

composer.json

@@ -24,7 +24,7 @@
         "php": ">=7.2",
         "ext-pdo_sqlite": "*",
         "symfony/symfony": "^4.4",
-        "symfony/flex": "^1.0",
+        "symfony/flex": "^2.4",
         "symfony/framework-bundle": "^4.4",
         "symfony/monolog-bundle": "^3.1,<3.7",
         "broadway/broadway-bundle": "~0.5",