Support smooth upgrade #2843
Description
Is your feature request related to a problem? Please describe.
After reading the upgrade documentation, I was frustrated,
I found that he doesn't support smooth upgrades, which I think is a big flaw in the feature. I think we should support non-stop upgrades that，
When using k8s, we will consider deploying different services inside a cluster if we want to
If we have to migrate the whole cluster for agones upgrade, I think this may be a drawback. In order to use agones, we have to build a cluster dedicated to agones, so that we can avoid migrating other applications and other services when upgrading agones.
The cluster where agones is located it has a lot of things, such as monitoring, persistence, host optimization, etc., then these means that it needs to be done again, although there are quite a few tools to simplify this part of the work. But I think this is still unnecessary. That's why we should support smooth upgrades. Avoid migrating clusters in a way.
Describe the solution you'd like
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Describe alternatives you've considered
Perhaps, the way to upgrade we can refer to istio
Additional context
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