Slackdiff Hook Fails Due to Depreciation of files.upload API method

[edchiodo]

When attempting to send diffs to a Slack channel using the slackdiff hook, the following error is returned:

ERROR -- : Hook slack (#SlackDiff:0x00005613e0fc8498) failed (#<Slack::Web::Api::Errors::MethodDeprecated

I believe this is due to the retirement of the web API method files.upload for any Slack apps created after May 16, 2024, and will be sunset entirely on March 11, 2025: https://api.slack.com//changelog/2024-04-a-better-way-to-upload-files-is-here-to-stay

[robertcheramy]

Maybe related to #3144

[varesa]

Potential fix: #3268 (the "works for me" kind at least)