chore: refactor changelogs

[Eric-Warehime]

Description

• Remove all of the .changelog files--this includes the unclog automation.
• Updates the contribution docs to reflect the fact that contributors should place changelog updates directly in the CHANGELOG.md file.
• Updates the release process doc to describe the process of moving the UNRELEASED changes to a new release section.

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Author Checklist

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please add links to any relevant follow up issues.

I have...

• Included the correct type prefix in the PR title
• Targeted the correct branch (see PR Targeting)
• Provided a link to the relevant issue or specification
• Reviewed "Files changed" and left comments if necessary
• Confirmed all CI checks have passed

Reviewers Checklist

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I have...

• Confirmed the correct type prefix in the PR title
• Confirmed all author checklist items have been addressed
• Confirmed that this PR does not change production code

[vladjdk]

lgtm