Bump letter_opener from 1.7.0 to 1.8.1 (#17838)

Bumps [letter_opener](https://github.com/ryanb/letter_opener) from 1.7.0 to 1.8.1.
- [Release notes](https://github.com/ryanb/letter_opener/releases)
- [Changelog](https://github.com/ryanb/letter_opener/blob/master/CHANGELOG.md)
- [Commits](https://github.com/ryanb/letter_opener/compare/v1.7.0...v1.8.1

)

---
updated-dependencies:
- dependency-name: letter_opener
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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