chore: Slack file upload changes (#10903)

The Slack `files.upload` API endpoint is deprecated and will stop
functioning on March 11, 2025. In this PR, we have implemented the
changes for the [new file
upload](https://api.slack.com/messaging/files#uploading_files) method.

spec/lib/integrations/slack/send_on_slack_service_spec.rb

@@ -162,15 +162,13 @@ describe Integrations::Slack::SendOnSlackService do
         attachment = message.attachments.new(account_id: message.account_id, file_type: :image)
         attachment.file.attach(io: Rails.root.join('spec/assets/avatar.png').open, filename: 'avatar.png', content_type: 'image/png')
 
-        expect(slack_client).to receive(:files_upload).with(hash_including(
-                                                              channels: hook.reference_id,
-                                                              thread_ts: conversation.identifier,
-                                                              initial_comment: 'Attached File!',
-                                                              filetype: 'png',
-                                                              content: anything,
-                                                              filename: attachment.file.filename,
-                                                              title: attachment.file.filename
-                                                            )).and_return(file_attachment)
+        expect(slack_client).to receive(:files_upload_v2).with(
+          filename: attachment.file.filename,
+          content: anything,
+          channel_id: hook.reference_id,
+          thread_ts: conversation.identifier,
+          initial_comment: 'Attached File!'
+        ).and_return(file_attachment)
 
         message.save!