E2E tests for different feature gate states

[m1kola]

Now that we have this feature gate:

operator-controller/pkg/features/features.go

Line 19 in b8f3626

ForceSemverUpgradeConstraints: {Default: false, PreRelease: featuregate.Alpha},

We should find a way to run E2E in CI with different states of feature gates.

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One option is to use kustomze overlays for different configurations (potentially using components). However this means that we need one overlay for every permutation which might become a bit messy.

We can also look into Kubernetes tests and see what is done there and whether we can re-use any tools/code (we use k8s.io/component-base/featuregate package for feature gates, same as Kubernetes).

[m1kola]

It was suggested in #444 (comment) as an option to E2E default state and cover other states with fast tests (unit/integration tests).