Give the GITHUB_TOKEN permission to create PRs

[whi-tw]

What

Github allows you to elevate the provided GITHUB_TOKEN's permissions, so it can create a PR.

Rather than having an unprivileged bot token, elevate the token's permissions to allow this via the workflow.

(@AP-Hunt told me that there were issues with this that needed a workaround, and it's just come up again on https://github.com/alphagov/paas-docker-cloudfoundry-tools/pull/226)

How to review

Code review

---

🚨⚠️ Please do not merge this pull request via the GitHub UI ⚠️🚨