[WIP] Remove Python 3.8 Support

[jrmccluskey]

Python 3.8 is scheduled to reach EOL in October of 2024. This PR removes support for Python 3.8 ahead of the Beam 2.60.0 release, following versioning changes in #27194 and #30828.

DO NOT MERGE

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.96%. Comparing base (8601bba) to head (78ae8ff). Report is 144 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #32279   +/-   ##
=========================================
  Coverage     77.96%   77.96%           
  Complexity     3023     3023           
=========================================
  Files           750      750           
  Lines         96303    96287   -16     
  Branches       3301     3301           
=========================================
- Hits          75084    75074   -10     
+ Misses        19702    19696    -6     
  Partials       1517     1517           

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