!fix(gov): duplicate registered amino path

[tbruyelle]

Description

Closes: #43

Replace cosmos-sdk prefix with govgen.

Marked as "client breaking" because old client won't be able to broadcast transactions with the amino-json signing mode.

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Author Checklist

All items are required. Please add a note to the item if the item is not applicable and please add links to any relevant follow up issues.

I have...

• [x] Included the correct type prefix in the PR title
• [x] Added ! to the type prefix if API, client, or state breaking change (i.e., requires minor or major version bump)
• [x] Targeted the correct branch (see PR Targeting)
• [x] Provided a link to the relevant issue or specification
• [ ] Followed the guidelines for building SDK modules
• [ ] Included the necessary unit and integration tests
• [x] Added a changelog entry in .changelog (for details, see contributing guidelines)
• [ ] Included comments for documenting Go code
• [ ] Updated the relevant documentation or specification
• [ ] Reviewed "Files changed" and left comments if necessary
• [ ] Confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add your handle next to the items reviewed if you only reviewed selected items.

I have...

• [ ] confirmed the correct type prefix in the PR title
• [ ] confirmed ! in the type prefix if API or client breaking change
• [ ] confirmed all author checklist items have been addressed
• [ ] reviewed state machine logic
• [ ] reviewed API design and naming
• [ ] reviewed documentation is accurate
• [ ] reviewed tests and test coverage