Make `MsgConnOpenTry.PreviousConnectionId` a reserved field

[colin-axner]

Summary

It's deprecation notice came in v4. I think we can reserve it now (assuming e2e's pass)

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[gjermundgaraba]

Considering we want to deprecate all of IBC v1, we will not spend any time doing this.

[gjermundgaraba]

This issue was mistakenly reopened during some integration work, so closing again now.