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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 30.43%. Comparing base (12ee6cb) to head (bb7ad65).
⚠️ Report is 57 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1791      +/-   ##
==========================================
- Coverage   39.44%   30.43%   -9.01%     
==========================================
  Files         287      287              
  Lines       22706    22074     -632     
==========================================
- Hits         8956     6719    -2237     
- Misses      12777    14626    +1849     
+ Partials      973      729     -244     

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[stefanprodan]

I think is Ok to check the Accepted condition. If the Gateway API implementation does set this condition, then it should properly reflect the observed generation.