Remove function `GetTimestamp` of `ConsensusState` interface

[crodriguezvega]

This function has been marked as deprecated in #5353, so now it just needs to be removed. It would seem natural to me to do this in v9, together with the 02-client routing refactoring, so that's why I am adding it to that milestone. Since v9.0.0 will be the first major release where the deprecation notice is given, then we should remove it in v11.

[neitdung]

Hi @crodriguezvega, could I take this PR for now, or do we need to wait until v9 is released?

[crodriguezvega]

Hi @crodriguezvega, could I take this PR for now, or do we need to wait until v9 is released?

Thank you for reaching out, @neitdung. We have decided to not remove it yet, so this issue can still wait. v9 will be the first major version where the deprecation notice will be effective, and our policy is to wait at least for two major releases before remove deprecated code.