fix: deprecated slack files.upload is replaced with new slack bot APIs

[iamatulsingh]

ENHANCEMENT

The original web API method for uploading files to Slack, [files.upload](https://api.slack.com/methods/files.upload), is being sunset on March 11, 2025.

As of May 16, 2024, newly-created apps are no longer able to use this API method.

Existing apps & integrations should migrate away from files.upload and instead leverage a combination of two APIs: [files.getUploadURLExternal](https://api.slack.com/methods/files.getUploadURLExternal) and [files.completeUploadExternal](https://api.slack.com/methods/files.completeUploadExternal). The use of these two methods is more reliable, especially when uploading large files.

A detailed breakdown of how to use these two API methods is described in our [Uploading files documentation](https://api.slack.com/messaging/files#uploading_files).

Mentioned in some issues #235

As mentioned here https://api.slack.com/changelog/2024-04-a-better-way-to-upload-files-is-here-to-stay I mad changes to use new APIs now with backward compatibility for Slack.

[CLAassistant]

All committers have signed the CLA.

[iamatulsingh]

Thank you for working on this!

It's not commonplace in Grafana to reach into environment variables directly, so we'll need to find an alternative to this, some options:

1. Don't make it configurable, just use the new API and be done with it.
2. Make it configurable via Grafana global config level, pass the config down
3. Make it configurable on the integration level

Thank you for your time. I have used the new API and removed the old one completely.