Add docs on how to pin gitlabktl to a specific version

Problem to solve

As a DevOps Engineer, in order to make sure my builds will never fail unexpectedly, I would like to pin the gitlabktl version of my builds.

Intended users

• Devon (DevOps Engineer)

Documentation

Document how to pin gitlabktl to a specific version.

Testing

Links

This is an issue to deprecate Knative 0.5 support in a nice way, related to gitlabktl#55 (closed)