docs: Add "deprecated" section to changelog

[roypat]

We want to separate these from the "changes" section.

Changes

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Reason

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License Acceptance

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PR Checklist

• If a specific issue led to this PR, this PR closes the issue.
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• API changes follow the Runbook for Firecracker API changes.
• User-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.
• New TODOs link to an issue.
• Commits meet contribution quality standards.

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• This functionality cannot be added in rust-vmm.

[pb8o]

We may also want a "Removed" section in the future. Not sure if it's worth adding now.

[roypat]

We may also want a "Removed" section in the future. Not sure if it's worth adding now.

Mh, yeah, we will definitely want it for major releases. Not sure if it makes sense to include in the template, given that for non-major releases it wont be used though