slack api change, files.upload will be discontinued 2025-03-11

[mtfurlan]

files.upload is being replaced by files.getUploadURLExternal and files.completeUploadExternal

You can see the post about it here: https://api.slack.com/changelog/2024-04-a-better-way-to-upload-files-is-here-to-stay

As of May 16, 2024, newly-created Slack apps are no longer able to access the files.upload API. Existing apps created prior to May 16, 2024 will be able to continue using files.upload until March 11, 2025.

looks like we just use UploadFileV2 not UploadFile from slack-go/slack, but according to https://github.com/slack-go/slack/issues/1171 the api is a little different