Automate first part of release

[nataliecarey]

Context

Currently the release steps are as documented here Releasing instructions
https://github.com/alphagov/govuk-prototype-kit/blob/v12.0.2/internal_docs/releasing.md

Steps 1-7 create a PR, steps 8-11 publish the release and step 12 shares the information on Slack.

Alternatives

We could automate this process in parts, starting with steps 1-7. If we get that part of the process wrong then there'll be nothing more than a Pull Request we won't ever merge.

[joelanman]

@nataliecarey shall we close this as v13 won't use the same release process