fix: tx compat

[technicallyty]

Description

Closes: #23846

This PR fixes compatibility such that a v0.53 client can send transactions to v0.50 chains. It does so by ensuring that transaction builders (aux builder, etc) explicitly set timestamppb.Timestamp to nil when the time.Time value is the zero/default value.

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[github-actions]

@technicallyty your pull request is missing a changelog!

[aaronc]

Seems like we should update the PR title and description since the TxBodyCompat type isn't actually needed anymore, right? It would be good to document the workaround and potential gotchas in detail in code comments in the relevant places

[aljo242]

lgtm