chore(multichain-account-service): fix changelog

[ccharly]

Explanation

Changelog got mixed up somehow, just fixing it.

References

N/A

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Low Risk
Documentation-only change that reorders/restores the 7.1.0 release notes in CHANGELOG.md; no runtime code is affected.

Overview
Fixes packages/multichain-account-service/CHANGELOG.md where the 7.1.0 section had been accidentally embedded/disabled in the Unreleased entries.

Restores the proper ## [7.1.0] heading and its Added/Changed items as a distinct released section so the changelog renders correctly.

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