Allow triggering of update process via GitHub Actions

[iansltx]

Instead of needing to set things up locally, we should have a manually triggered GitHub action (see https://phpc.social/@ian/110442635025636470) to perform the same actions as a production deploy would take locally. Theoretically we could automate this later to happen on a push to either web2 or api repos, but my guess is that's a disproportionate amount of effort for a minor convenience gain, and we'd likely want to approve deploys anyway so this feels like a happy medium.