Automatically trigger the release workflow on merge of the "Prepare release" PR

[edmorley]

Currently the steps to perform a CNB release are:

1. Trigger the "Prepare release" workflow, and wait for it to open the "Prepare release PR"
2. Leave a review approval on the CNB release PR (the PR will then auto-merge)
3. Trigger a "Release buildpack" workflow targetting main, and wait for it to perform the release and eventually open the builder repo PR
4. Leave a review approval on the builder update PR (the PR will then auto-merge)

As a future optimisation, it seems we could skip step (3) by having the workflow auto-trigger when the "Prepare release" PR is merged.

The SF CLI has a workflow that does something similar:
https://github.com/salesforcecli/cli/blob/8aa0ab825cc4e7367690c0baa6011ee6134dc6b7/.github/workflows/create-github-release.yml#L4-L17

GUS-W-13871118.