Scripts: update buildnumber to use timezone-aware datetime

[onetechnical]

Summary

Running make will get this deprecation warning for newer versions of python:

./scripts/buildnumber.py:13: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
  d1 = datetime.utcnow()

This is because utcnow is timezone-aware, and the earlier datetime call is naive. To make this consistent, we just mark all to UTC, to make consistent with the comment for setting the epoch.

Test Plan

Verified that ./scripts/buildnumber.py returned output without deprecation warnings.

Verified on 3.12.2 and 3.8.10 (version shipped with Ubuntu 20.04).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.72%. Comparing base (0e7087b) to head (0b5e635).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5971      +/-   ##
==========================================
- Coverage   55.73%   55.72%   -0.01%     
==========================================
  Files         490      490              
  Lines       68125    68125              
==========================================
- Hits        37967    37963       -4     
- Misses      27586    27589       +3     
- Partials     2572     2573       +1     

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