Bump symfony/yaml from 3.4.2 to 4.4.9

Bumps [symfony/yaml](https://github.com/symfony/yaml) from 3.4.2 to 4.4.9.
- [Release notes](https://github.com/symfony/yaml/releases)
- [Changelog](https://github.com/symfony/yaml/blob/master/CHANGELOG.md)
- [Commits](https://github.com/symfony/yaml/compare/v3.4.2...v4.4.9)

Signed-off-by: dependabot[bot] <support@github.com>

composer.json

@@ -55,7 +55,7 @@
         "doctrine/instantiator": "^1.1",
         "phpdocumentor/reflection-docblock": "3.3.2",
         "symfony/debug": "3.4.2",
-        "symfony/yaml": "3.4.2",
+        "symfony/yaml": "4.4.9",
         "symfony/finder": "3.4.8",
         "symfony/stopwatch" : "4.4.9",
         "myclabs/deep-copy": "1.9.5",