[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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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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