Update release script to update all workspace package.jsons

[Caleb-Ellis]

When releasing a new version of maas-ui, we need to update the version number in 10 places across package.json, legacy/package.json, proxy/package.json, root/package.json, shared/package.json and ui/package.json. It would be nice to automate this process as it can be easy to forget something.

This should probably include an update to RELEASE.md to reflect the changes.

[cristinadresch]

this will be fixed by upgrading yarn