Replace deprecated Slack API method in `SlackAPIFileOperator` (`files.upload`)

[shahar1]

Description

files.upload method is deprecated and will soon be removed on November 12, 2025, which will make SlackAPIFileOperator practically unusable.
It is suggested to use the combination of files.getUploadURLExternal and files.completeUploadExternal instead.

Use case/motivation

No response

Related issues

No response

Are you willing to submit a PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct

[shahar1]

Already solved in #54061 :)