This pull request adds a deprecation notice for files.upload and addresses #1287.
files.upload is deprecated and will stop functioning on March 11, 2025. Use files.getUploadURLExternal and files.completeUploadExternal to upload files instead. Newly created apps will be unable to use files.upload beginning May 8, 2024. See Uploading files for more details on the process and this changelog for more on the deprecation.
This pull request adds a deprecation notice for files.upload and addresses #1287.
Reference: https://api.slack.com/methods/files.upload#markdown