chore(docs): Add NOTE to direct ASM users using GKEHub TF Module

[wonjekang]

Currently, ASM Submodule is not supported / managed anymore by ASM team. Highly recommending customers using GKEHub TF module to install ASM.

[wonjekang]

@bharathkkb I think adding this context in the migration guide is a good idea. Do we have a timeline to have it? However, we still have ASM customers depend on this ASM submodule and we cannot remove it at this time. Let's keep this change at this time.

[bharathkkb]

I think adding this context in the migration guide is a good idea. Do we have a timeline to have it?

If we are recommending to switch to the new resources, co-locating the migration guide in the PR would be good. Otherwise let's make it a fast follow.

We still have ASM customers depend on this ASM submodule and we cannot remove it at this time. Let's keep this change at this time.

Even if we remove the module, users can still use continue to use released versions. Only new module releases will not have this submodule.

[wonjekang]

@bharathkkb Could you share the instruction to write the migration guide? I don't have any context where I can start from. IIUC, your recommendation in this PR is:

1. Remove the ASM submodule package.
2. Add the context to use gkehub resources for ASM installation in migration guide document(?)

[bharathkkb]

@wonjekang Yeah for 1 and 2. Example guides are in https://github.com/terraform-google-modules/terraform-google-kubernetes-engine/tree/master/docs and next major version would be 31. Here is a previous ASM example https://github.com/terraform-google-modules/terraform-google-kubernetes-engine/blob/master/docs/upgrading_to_v20.0.md#upgrading-to-v200