ui: require yarn >= 1.7.0

[jseldess]

PR: cockroachdb/cockroach#27262

From release notes:

Building CockroachDB from source now requires yarn at version 1.7.0 or above. [#27262][#27262] {% comment %}doc{% endcomment %}

[Amruta-Ranade]

As discussed with @benesch, this issue doesn't require doc updates because the documented procedure to build from source doesn't require yarn. Closing this issue.

[jseldess]

Thanks, @Amruta-Ranade. For this type of update, we also always want to check the cockroach CONTRIBUTING.md. That's where our most explicit build from source instructions are. But it turns out that the yarn requirement is already up-to-date there as well!

[Amruta-Ranade]

@jseldess Oh! Didn't know that. Made a note of it for next time :)