py/makeversionhdr.py: Fix datetime.utcfromtimestamp() is deprecated.

[Josverl]

Summary

CI for the reproducible build shows a deprecation warning.

mkdir -p build-minimal/shared/timeutils/
/home/runner/work/micropython/micropython/ports/unix/../../py/makeversionhdr.py:120: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
  build_date = datetime.datetime.utcfromtimestamp(

https://github.com/micropython/micropython/actions/runs/14346813947/job/40218094016

I updated py/makeversionhdr.py to use datetime.datetime.fromtimestamp(timestamp, datetime.timezone.utc) instead

Testing

Tested minimal and standard unix build outside of CI
CI will test the rest

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (aedaa40) to head (1b12357).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #17099   +/-   ##
=======================================
  Coverage   98.54%   98.54%           
=======================================
  Files         169      169           
  Lines       21898    21898           
=======================================
  Hits        21580    21580           
  Misses        318      318           

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[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32