fix: gateway router should wait for accepted condition

[steved]

It can take some time for changes to propagate for cloud load balancers, so flagger should ensure the route changes are current before proceeding with any more.

I'm not considering observedGeneration optional due to the upstream language, but I'm not sure if it's truly guaranteed so if it's easier to make this behavior opt-in then I'm happy to add it.

Additionally, the upstream Conditions struct contains an optional observedGeneration field - implementations MUST use this field and set it to the metadata.generation field of the object at the time the status is generated. This allows users of the API to determine if the status is relevant to the current version of the object.

https://github.com/kubernetes-sigs/gateway-api/blob/v1.2.1/site-src/guides/implementers.md?plain=1#L156C1-L160C23