Enable GatewayClassObservedGenerationBump conformance test

[arkodg]

Description:
Seeing this conformance test fail in in #780

Once fixed it should be enabled via https://github.com/envoyproxy/gateway/blob/main/test/conformance/conformance_test.go

[danehans]

This is the last 0.6.0 conformance test that is failing.

[Xunzhuo]

See: https://kubernetes.slack.com/archives/CR0H13KGA/p1673599220709819.

[danehans]

Waiting for gw api v0.6.1 to drop.