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
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