docs(release): generate 3.8.1 changelog

[kevwilliams]

Summary

Updating changelog for 3.8.1

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

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All committers have signed the CLA.

[CLAassistant]

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