slack files.upload deprecated, migrate to new upload method

[mcdamo]

Slack files.upload was deprecated on May 16, 2024 and will be retired on March 11, 2025.

This change uses the new upload method as detailed here

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.

Project coverage is 49.45%. Comparing base (9d61fc5) to head (57928d0).

Files with missing lines	Patch %	Lines
mqttwarn/services/slack.py	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #701      +/-   ##
==========================================
- Coverage   49.75%   49.45%   -0.30%     
==========================================
  Files          81       81              
  Lines        4034     4030       -4     
==========================================
- Hits         2007     1993      -14     
- Misses       2027     2037      +10     

Flag	Coverage Δ
unittests	49.45% <0.00%> (-0.30%)	⬇️

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[amotl]

Hi @mcdamo, thanks a stack for your contribution. If @jpmens or @sumnerboy12 don't have any specific objections, let's merge your patch without much ado, and run another release?