fix: Upgrade letter_opener to fix cannot load such file -- kconv error (#11809)

The email jobs were failing in local development due to a cannot load
such file -- kconv error. This was caused by a removed dependency in the
latest version of the letter_opener gem. Upgrading to the latest version
resolves the issue.

Gemfile.lock

@@ -172,6 +172,8 @@ GEM
       bundler (>= 1.2.0, < 3)
       thor (~> 1.0)
     byebug (11.1.3)
+    childprocess (5.1.0)
+      logger (~> 1.5)
     climate_control (1.2.0)
     coderay (1.1.3)
     commonmarker (0.23.10)
@@ -433,10 +435,12 @@ GEM
       json (>= 1.8)
       rexml
     language_server-protocol (3.17.0.5)
-    launchy (2.5.2)
+    launchy (3.1.1)
       addressable (~> 2.8)
-    letter_opener (1.8.1)
-      launchy (>= 2.2, < 3)
+      childprocess (~> 5.0)
+      logger (~> 1.6)
+    letter_opener (1.10.0)
+      launchy (>= 2.2, < 4)
     line-bot-api (1.28.0)
     lint_roller (1.1.0)
     liquid (5.4.0)
@@ -563,7 +567,7 @@ GEM
       method_source (~> 1.0)
     pry-rails (0.3.9)
       pry (>= 0.10.4)
-    public_suffix (6.0.0)
+    public_suffix (6.0.2)
     puma (6.4.3)
       nio4r (~> 2.0)
     pundit (2.3.0)