feat: Add support for grouped file uploads in Slack (#12454)

Fixes
https://linear.app/chatwoot/issue/CW-5646/add-support-for-grouped-file-uploads-in-slack

Previously, when sending multiple attachments to Slack, we uploaded them
one by one. For example, sending 5 images would result in 5 separate
Slack messages. This created clutter and a poor user experience, since
Slack displayed each file as an individual message.
This PR updates the implementation to group all attachments from a
message and send them as a single Slack message. As a result,
attachments now appear together in one grouped block, providing a much
cleaner and more intuitive experience for users.

**Before:** 
Each file uploaded as a separate Slack message.
<img width="400" height="800" alt="before"
src="https://github.com/user-attachments/assets/c8c7f666-549b-428f-bd19-c94e39ed2513"
/>

**After:** 
All files from a single message grouped and displayed together in one
Slack message (similar to how Slack natively handles grouped uploads).
<img width="400" height="800" alt="after"
src="https://github.com/user-attachments/assets/0b1f22d5-4d37-4b84-905a-15e742317e72"
/>

**Changes**

- Upgraded Slack file upload implementation to use the new multiple
attachments API available in slack-ruby-client `v2.7.0`.
- Updated attachment handling to upload all files from a message in a
single API call.
- Enabled proper attachment grouping in Slack, ensuring related files
are presented together.

Gemfile

@@ -103,7 +103,7 @@ gem 'twitty', '~> 0.1.5'
 # facebook client
 gem 'koala'
 # slack client
-gem 'slack-ruby-client', '~> 2.5.2'
+gem 'slack-ruby-client', '~> 2.7.0'
 # for dialogflow integrations
 gem 'google-cloud-dialogflow-v2', '>= 0.24.0'
 gem 'grpc'