Add a migration to push the start date to be later

[p-offtermatt]

Description

Closes: N/A

Add a description of the changes that this PR introduces and the files that
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Adds a migration to push the start date to be later.
Concretely:
the migrate message will contain the new first round start date.
The contract will simply set its start date to that date, and set the end of all existing locks to the end of the new first round.

The migration has a bunch of checks to make sure its assumptions are correct:

• it only works while the contract is in its first round
• it only works if the new first round end is in the future

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Author Checklist

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I have...

• Targeted the correct branch
• Included the necessary unit tests
• [o] Added/adjusted the necessary interchain tests
• [o] Added a changelog entry in .changelog
• Compiled the contracts by using make compile and included content of the artifacts directory into the PR
• Regenerated front-end schema by using make schema and included generated files into the PR
• Updated the relevant documentation or specification
• Reviewed "Files changed" and left comments if necessary
• Confirmed all CI checks have passed

[dusan-maksimovic]

Missing changelog, otherwise good.