Upgrade slack-messenger gem to fix Ruby 2.7 deprecation warnings

This pulls in the changes from
https://gitlab.com/gitlab-org/slack-notifier/-/merge_requests/5.

Gemfile

@@ -244,7 +244,7 @@ gem 'atlassian-jwt', '~> 0.2.0'
 gem 'flowdock', '~> 0.7'
 
 # Slack integration
-gem 'slack-messenger', '~> 2.3.3'
+gem 'slack-messenger', '~> 2.3.4'
 
 # Hangouts Chat integration
 gem 'hangouts-chat', '~> 0.0.5'

Gemfile.lock

@@ -1096,7 +1096,7 @@ GEM
       simplecov (~> 0.8)
     simplecov-html (0.12.2)
     sixarm_ruby_unaccent (1.2.0)
-    slack-messenger (2.3.3)
+    slack-messenger (2.3.4)
     snowplow-tracker (0.6.1)
       contracts (~> 0.7, <= 0.11)
     spring (2.0.2)
@@ -1471,7 +1471,7 @@ DEPENDENCIES
   simple_po_parser (~> 1.1.2)
   simplecov (~> 0.18.5)
   simplecov-cobertura (~> 1.3.1)
-  slack-messenger (~> 2.3.3)
+  slack-messenger (~> 2.3.4)
   snowplow-tracker (~> 0.6.1)
   spring (~> 2.0.0)
   spring-commands-rspec (~> 1.0.4)