Replace discontinued files.upload API in Slack platforms

[mszostok]

Overview

The files.upload web API method that we use to upload a long messages (e.g. logs) will be discontinued.

To prepare for this change, we recommend migrating away from files.upload and instead using the combination of files.getUploadURLExternal and files.completeUploadExternal. More information on how to use these two APIs together can be found on our Uploading files documentation. You can also leverage Slack’s SDKs to help you transition to this new way of uploading files by visiting our api.slack page.

Acceptance Criteria

• Update Slack platforms to use new API
• Probably we need to contribute/fork/extend the library:
  • Upload Files V2: Ability to upload multiple files in one message slack-go/slack#1288
  • files.upload is getting deprecated by Slack slack-go/slack#1287

Reason

Support for the files.upload API will be discontinued on March 11, 2025.