Currently new projen development builds are failing (example: https://github.com/projen/projen/pull/1721) because `yaml` published v2.0.0 to the `next` tag, in which they bumped the minimum node version they require from 12 to 14. Since new development builds are automatically picking this up and projen still supports node v12, this PR pins the version of `yaml` so the new version isn't picked up by `yarn install`.

This should fix #1740

We should also upgrade the minimum node version projen supports, but it's better to do that in a separate PR. (Issue: https://github.com/projen/projen/issues/1739)

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