replace "datetime.utcfromtimestamp" to avoid deprecation warnings with Python 3.12

[FelixSchwarz]

datetime.utcfromtimestamp is deprecated in Python 3.12:

src/borg/testsuite/helpers.py:1218: DeprecationWarning: datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.fromtimestamp(timestamp, datetime.UTC).
    assert datetime.utcfromtimestamp(safe_ns(beyond_y10k) / 1000000000) > datetime(2038, 1, 1)

I added a custom helper with emulates the behavior of datetime.utcnow to avoid the deprecation warning. My idea was that creating tz-aware datetime instances creates longer code lines and reduces readability.

[codecov-commenter]

Codecov Report

Merging #7715 (ae0b3d2) into master (e695683) will decrease coverage by 0.32%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master    #7715      +/-   ##
==========================================
- Coverage   83.79%   83.47%   -0.32%     
==========================================
  Files          66       66              
  Lines       11893    11893              
  Branches     2153     2153              
==========================================
- Hits         9966     9928      -38     
- Misses       1356     1385      +29     
- Partials      571      580       +9     

see 8 files with indirect coverage changes