Upgrade Yarn to v1.6

[islemaster]

Second attempt at #21954.

Turns out the Yarn signing key was changed around v1.0.1 and existing servers that have a previous signing key are encountering an error when trying to update the apt-cache.

ubuntu@staging:~$ sudo apt-get update
[...]
Err https://dl.yarnpkg.com stable InRelease                                    
[...]
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://dl.yarnpkg.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E074D16EB6FF4DE3

W: Failed to fetch https://dl.yarnpkg.com/debian/dists/stable/InRelease  

W: Some index files failed to download. They have been ignored, or old ones used instead.

@wjordan found this reported as an issue on the Yarn repository. It looks like re-adding the key is sufficient to fix for now (curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -) but I haven't worked out the best way to make that happen once for servers that need updating in our chef recipe yet.

[wjordan]

The command to refresh keys in the Apt repository is: (as root or add sudo)

apt-key adv --refresh-keys --keyserver ${KEYSERVER}

(where KEYSERVER=ha.pool.sks-keyservers.net which is a pool of highly-available keyservers, or any other pgp keyserver)

I've manually refreshed the apt-repo keys in our CI pipeline which should resolve this issue for now. In the future we can look into automating this refresh with a cron task or something if it ever causes issues again.