Pin yarn in schemas package

[mikewilli]

We should probably not just grab the latest version of yarn for the schemas Dockerfile. If this is in fact what we want, a comment saying this is probably warranted.

Related: we should probably remove the versions for all the prereqs in the schemas README so that we don't have to constantly update them with changes like this. I just looked to see where else we might be saying a poetry version and found that:

• python says 3.11 which is wrong
• poetry says 1.2.2 which is OLD
• yarn says 1.22 but seems like the Dockerfile just grabs the latest?

Originally posted by @mikewilli in https://github.com/mozilla/experimenter/pull/11646#pullrequestreview-2405412929

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