Bump letter_opener from 1.4.1 to 1.6.0

Bumps [letter_opener](https://github.com/ryanb/letter_opener) from 1.4.1 to 1.6.0.
- [Release notes](https://github.com/ryanb/letter_opener/releases)
- [Changelog](https://github.com/ryanb/letter_opener/blob/master/CHANGELOG.md)
- [Commits](ryanb/letter_opener@v1.4.1...v1.6.0)

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

Author: dependabot-support

Gemfile.lock

@@ -200,7 +200,7 @@ GEM
     jwt (1.5.2)
     launchy (2.4.3)
       addressable (~> 2.3)
-    letter_opener (1.4.1)
+    letter_opener (1.6.0)
       launchy (~> 2.2)
     loofah (2.2.2)
       crass (~> 1.0.2)