Align release process with that of GOV.UK Frontend

[romaricpascal]

Following this morning's release, I've tried to update the release process to bring it even closer to how we release GOV.UK Frontend.

The steps were already following a very similar process. The main technical change is that instead of having the commit and tag generated by npm version, like it was in the current process, it is now handled manually (similarly to how the release scripts of GOV.UK Frontend commit the version change and create a tag).

I've also updated the draft for the comms to match what we did in the latest release.