Update ICS27 documentation for ibc-go/v6

[seantking]

Summary

Update ICS27 documentation with v6 changes.

• Document addition of ICS27 controller MsgServer
  • MsgRegisterInterchainAccount
  • MsgSendTx
  • Usage of MsgServer routing by custom application modules
• Deprecation of Go APIs and nil underlying applications.

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[colin-axner]

Could you open an issue on the spec repo as well? Also, what is the preferred way to reference the controller module within this repo when compared against a controller module

[seantking]

Could you open an issue on the spec repo as well?

I don't think we reference authentication modules in the spec. I'll have a look.

Also, what is the preferred way to reference the controller module within this repo when compared against a controller module

I think we can just refer to our controller module as the "base" controller module. wdyt?

[crodriguezvega]

Consider opening smaller issues if there are too many things to change.