bump vendored docker login plugin

we're preparing to release v5.0.0 and would like the 3 vendored plugins
to be up to date.

The full diff is
[here](buildkite-plugins/docker-login-buildkite-plugin@4703520...cb92aa6).
It's mostly docs, and non-functional updates.

The one exception is that support for multiple registries has been
removed. However, the way we use the embedded plugin only supports a single
registry. Here's how we describe using it in the stack README::

> If you want to push or pull from registries such as Docker Hub or Quay you
> can use the environment hook in your secrets bucket to export the following
> environment variables:
>
>     DOCKER_LOGIN_USER="the-user-name"
>     DOCKER_LOGIN_PASSWORD="the-password"
>     DOCKER_LOGIN_SERVER="" - optional. By default it will log into Docker Hub
>
> Setting these will perform a docker login before each pipeline step is run,
> allowing you to docker push to them from within your build scripts

Author: yob