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
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile

@@ -129,7 +129,7 @@ group :development do
   gem 'better_errors', '~> 2.9'
   gem 'binding_of_caller', '~> 1.0'
   gem 'bullet', '~> 7.0'
-  gem 'letter_opener', '~> 1.7'
+  gem 'letter_opener', '~> 1.8'
   gem 'letter_opener_web', '~> 2.0'
   gem 'memory_profiler'
   gem 'rubocop', '~> 1.26', require: false