docs: update supported k8s version

[amazingandyyy]

We have many issues opened regarding the compatible issue of argo-rollouts and k8s. Based on @zachaller comment on this #2718 issue, I think we all agree:

1. the old docs is unclear and not true, argo-rollouts doesn't only support k8s N and N-1 version, sometime it support N-3 version
2. the code is truth; as long as it's tested, we should be able to say argo-rollouts X version is compatible with k8s Y version

Resolves #2718

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional with a list of types and scopes found here, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed my commits with DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My builds are green. Try syncing with master if they are not.
• My organization is added to USERS.md.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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[zachaller]

Thanks for the pr

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (0b81ca2) 81.74% compared to head (e010a5b) 81.74%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2949   +/-   ##
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  Coverage   81.74%   81.74%           
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  Files         134      134           
  Lines       20370    20370           
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  Hits        16652    16652           
  Misses       2861     2861           
  Partials      857      857           

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[github-actions]

Go Published Test Results

2 040 tests   2 040 ✔️  2m 39s ⏱️
   118 suites         0 💤
       1 files           0 ❌

Results for commit e010a5b.