DEP Update symfony/yaml dependency

[GuySartorelli]

Fixes composer dependency problems in PHP linting job for silverstripe/silverstripe-behat-extension:
https://github.com/silverstripe/silverstripe-behat-extension/runs/7350017746?check_suite_focus=true

Job is failing because silverstripe/silverstripe-behat-extension requires behat/behat ^3.9 which in turn requires symfony/yaml >= 4

When merged, tag as 2.2.1 or 2.3.0 so it can be installed in CI under the ~2.0 constraint.

See also

• DEP Update symfony/yaml dependency #218

Parent issue:

• Broken builds from initial github actions CI config PRs gha-ci#37