chore(deps): bump @typespec/openapi3 from 1.6.0 to 1.8.0 in /additions (#993)

Bumps [@typespec/openapi3](https://github.com/microsoft/typespec) from 1.6.0 to 1.8.0.
- [Release notes](https://github.com/microsoft/typespec/releases)
- [Commits](https://github.com/microsoft/typespec/compare/typespec-stable@1.6.0...typespec-stable@1.8.0)

---
updated-dependencies:
- dependency-name: "@typespec/openapi3"
  dependency-version: 1.8.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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