docs: update ADR-064

[alexanderbez]

Description

As I've started Vote Extension implementation, I realized there is much simpler approach to vote extension state persistence, so I've documented this in the ADR.

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Author Checklist

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I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
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• confirmed the correct type prefix in the PR title
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• 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
• manually tested (if applicable)

[samricotta]

small change but other than that looks good