WIP: Update docs on updating clusterctl API version upgrade tests

[willie-yao]

What type of PR is this? /kind documentation

What this PR does / why we need it: This PR updates documentation on how to update the clusterctl API version upgrade tests on a new release. Previously the docs were pointing to the wrong place for updating versions, as well as not including guidance on how to update the cluster-template-prow.yaml that the test was using.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #

Special notes for your reviewer:

• [ ] cherry-pick candidate

TODOs:

• [ ] squashed commits
• [ ] includes documentation
• [ ] adds unit tests

Release note:

NONE