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

See also

• DEP Update symfony/yaml dependency #219

Parent issue:

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

[emteknetnz]

@GuySartorelli still failing after rerun - https://github.com/silverstripe/silverstripe-behat-extension/runs/7404200666?check_suite_focus=true