Switch to slack upload file v2 api

[vaijab]

Description

Changes proposed in this pull request:

• Switch to slack upload file v2 api

Testing

Related issue(s)

[madebyrogal]

@vaijab how can I test this?

[TonyLovesDevOps]

Subscribed to this PR... this is required as new apps won't work since slack deprecated files.upload for new apps.

I added botkube to a third cluster and it received an error like this when trying to return content:

{"bot":"SocketSlack","commGroup":"default-group","error":"while sending message: while uploading file: method_deprecated","level":"error","msg":"Failed to handle Socket Slack message","time":"2024-08-23T20:44:06Z"}

The other two clusters are still working since I created their corresponding Slack apps before the May 8, 2024 deadline. In either case, this method will stop working for all apps as of March 11, 2025.

[mszostok]

Will be fixed in #1480