chore(deps): bump @typespec/openapi from 1.6.0 to 1.8.0 in /additions

Bumps [@typespec/openapi](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/[email protected]@1.8.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]