Minor change to `update_compat_info.php`

Summary: Replace `file_put_contents` with `Filesystem::writeFile`.

Test Plan: Ran `./scripts/update_compat_info.php`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9960