chore: adding a warning message about Python 3.8

[sevbch]

Python 3.8 is no longer supported by the Python Software Foundation. Thus, GGShield will soon require Python 3.9 and above to run.

In this PR we add a deprecation warning message to warn Python 3.8 users about future deprecation.

Output example:

$ ggshield api-status
API URL: https://dashboard.gitguardian.com
Status: healthy
App version: v2.119.0
Secrets engine version: 2.126.0

Instance source: default
API key source: user config

Warning: Python 3.8 is no longer supported by the Python Software Foundation. GGShield will soon require Python 3.9 or above to run.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.04%. Comparing base (035d74b) to head (ae62e54).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1019   +/-   ##
=======================================
  Coverage   92.04%   92.04%           
=======================================
  Files         181      181           
  Lines        7717     7719    +2     
=======================================
+ Hits         7103     7105    +2     
  Misses        614      614           

Flag	Coverage Δ
unittests	92.04% <100.00%> (+<0.01%)	⬆️

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[agateau-gg]

Looks good now 👍🏻.

That was a bit more involved than I expected 😞.