Update deprecated datetime call

[nateinaction]

Related Issue(s)
Has Unit Tests (y/n)	n
Documentation Included (y/n)	n

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Change Description

This PR is updates a deprecated call to the datetime library.

Rationale

Fixing warnings:

FprimeZephyrReference/test/int/rtc_test.py::test_04_sequence_cancellation_on_time_set
  /Users/nate/code/github.com/open-source-space-foundation/proves-core-reference/fprime-venv/lib/python3.13/site-packages/fprime_gds/common/files/helpers.py:150: 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).
    self.__start = datetime.datetime.utcnow()

Testing/Review Recommendations

This does is not expected to change any behavior and compatible with Python 3.9+ which is the stated supported versions in the pyproject.toml

Future Work

Note any additional work that will be done relating to this issue.