Slack API: files.upload method is being deprecated

[naamayavor]

Hi, I'm a user of the slackr package and received this deprecation notice from Slack:

"Action required: Important notice about the files.upload API You are receiving this message because we have detected that your app has recently made a call to the files.upload API. We want to remind you that on November 12, 2025, support for this API will be discontinued. Please prepare for this change by moving away from using files.upload and instead use the combination of files.getUploadURLExternal and files.completeUploadExternal. More information on how to use these two APIs together can be found on our Working with 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. If you choose to not take action before November 12, 2025, and your app continues to attempt to use files.upload, your app may break or become unusable."

It looks like slackr_upload will need to be updated to use the new files.getUploadURLExternal and files.completeUploadExternal methods before November 12, 2025. Just wanted to bring this to your attention. Thanks for maintaining this great package!