Test upstream versions of Gateway API and Istio

[averevki]

Summary

Currently the testsuite only runs against stable versions of Gateway API and Istio, supported by OCP or OSSM. We could add a new testing path for their latest upstream versions to detect incompatibilities early and cover versions that upstream Kuadrant users may already be running.

Motivation

• Early detection: Problems with upcoming Gateway API or Istio releases are only discovered after those versions go GA, leaving little time to react.
• Upstream user coverage: Users running Kuadrant outside of OpenShift may use newer or unreleased versions of these dependencies. Testing against them gives confidence that Kuadrant works in those environments.

Proposed Approach

• Add a new configuration path in the testing matrix targeting latest/upstream builds of Gateway API and Istio (investigate for more dependencies that might require upstream testing).
• Run on a non-OpenShift Kubernetes environment — vanilla Kubernetes or the existing Kind cluster deployment would be a good fit, since upstream versions are more naturally consumed there.
• This shouldn't replace existing stable-version testing with OSSM and OCP