Remove python 3.8 for dbt 1.8

[peterallenwebb]

Problem

Python 3.8 is EOL and no longer supported by some of dbt's upstream dependencies.

Solution

We are ending support for 3.8 in all future releases, and will no longer test against it.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.25%. Comparing base (924ce2d) to head (949c16d).
Report is 1 commits behind head on 1.8.latest.

Files with missing lines	Patch %	Lines
core/setup.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           1.8.latest   #10869      +/-   ##
==============================================
- Coverage       88.33%   88.25%   -0.08%     
==============================================
  Files             181      181              
  Lines           22796    22796              
==============================================
- Hits            20136    20119      -17     
- Misses           2660     2677      +17     

Flag	Coverage Δ
integration	85.44% <0.00%> (-0.16%)	⬇️
unit	62.83% <0.00%> (ø)

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