Port release rake task

Port the rake task that automates release steps. This should probably be written in Go, similar to the migration scripts, with the same CLI framework (https://cobra.dev/).

• Rake task (Nanoc version): https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/lib/tasks/release.rake?ref_type=heads
• Release directions: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/.gitlab/issue_templates/release.md?ref_type=heads

The Nanoc script would do the following steps of the release.

• Create a branch for the new version
• Create a Dockerfile for the new version (copy single.Dockerfile and adjust the VER variable)
• Push the branch

The full process includes more steps, but these are still manual.

An improved version might do something like:

• Create the MR (https://gitlab.com/gitlab-org/cli/-/blob/main/docs/source/mr/create.md) from a template
• Create the Dockerfile
• Push it to the MR branch