chore(deps): upgrade twilio-ruby to 7.6.0 for upcoming features (#12243)

### Summary

- Update Twilio gem to support latest features and API changes.
- No app code changes; Gemfile and Gemfile.lock only.

references: #11602 , #11481 

### Testing

- Existing Twilio SMS: send/receive still works; delivery status
updates.
- Existing Twilio WhatsApp: send/receive still works; templates (if
used) unaffected.
- Create new Twilio SMS/WhatsApp inboxes: can be created and can
send/receive messages.

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

Gemfile

@@ -89,7 +89,7 @@ gem 'wisper', '2.0.0'
 ##--- gems for channels ---##
 gem 'facebook-messenger'
 gem 'line-bot-api'
-gem 'twilio-ruby', '~> 5.66'
+gem 'twilio-ruby'
 # twitty will handle subscription of twitter account events
 # gem 'twitty', git: 'https://github.com/chatwoot/twitty'
 gem 'twitty', '~> 0.1.5'