Add docs on how to pin gitlabktl to a specific version
### Problem to solve
<!-- What problem do we 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)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#devon-devops-engineer)
### Documentation
Document how to pin `gitlabktl` to a specific version.
### Testing
<!-- What risks does this change pose? How might it affect the quality of the product? What additional test coverage or changes to tests will be needed? Will it require cross-browser testing? See the test engineering process for further help: https://about.gitlab.com/handbook/engineering/quality/test-engineering/ -->
### Links
This is an issue to deprecate Knative 0.5 support in a nice way, related to https://gitlab.com/gitlab-org/gitlabktl/issues/55
issue