chore: make Assets the sole codeowner of core-backend

[Kriys94]

Explanation

References

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
Metadata-only ownership changes; no application or runtime code is modified.

Overview
core-backend is no longer jointly owned with Core Platform for day-to-day package changes. Ownership is consolidated under the Assets team in the same places other packages declare owners.

Updates codeowners.ts so core-backend lists only @MetaMask/metamask-assets, moves it from Joint team ownership into the Assets Team section, and regenerates .github/CODEOWNERS to match (including listing Assets first on release metadata paths). teams.json now maps metamask/core-backend to team-assets only, dropping team-core-platform.

Core Platform remains a co-owner on core-backend release files (package.json, changelogs, tsconfig, typedoc) via the existing “Package Release related” rules that always add that team.

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