Use multiple versions matrix for integration and e2e tests

[pmalek]

As of now KGO has no notion of using multiple k8s versions for integration or e2e tests like KIC does through e.g. Github Actions matrix and KONG_CLUSTER_VERSION env variable in e2e tests

While we do seem to be specifying the matrix for k8s versions (1 as of now)

• it uses an incorrectly named variable kubernetes-version vs kubernets_version
• it doesn't do anything else besides being used for job title

Proposed solution

In order to make our testing strategy more robust, we should at the very least at a handful of k8s versions for e2e tests as in KIC and propagate the information on which version to use in the tests themselves.

Action Items

• [ ] add all supported k8s versions to the e2e test suite runs
• [ ] add documentation indicating our support versions, as a product of our test results

[shaneutt]

This seems like something we would absolutely want to have prior to OSS Release v1, I've also updated the action items to request related documentation.