chore: MsgUpdateClient rename header to client_message

[damiannolan]

Description

• Renaming header field to client_message on MsgUpdateClient
• Updating field naming, protodocs, godocs
• Adding deprecation notices for misbehaviour submission handlers

closes: #XXXX

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[damiannolan]

I'm unsure if this was forgotten about or if this was intended to be left as header. If not required we can close this PR or take the deprecation notices to another PR.

My only concern here is that relayers would be required to support both field names simultaneous depending on what version a chain and it's counterparty are running. Afaik hermes queries chains to determine ibc-go versions already but I am not sure if they have had to support such a scenario yet.
Despite this I think we should still be allowed to freely rename such fields for major version releases.

Curious to hear the thoughts of the team! :)

[crodriguezvega]

Good question... If needed I can check with Adi and Justin.

[colin-axner]

I believe updating the naming should be fine. But we should check with relayers

[colin-axner]

ACK. Nice!

[seantking]

Yurt