fix(timezone): Make datetime usage timezone-aware for DTZ lint rules

[jenshnielsen]

• Use datetime.now(timezone.utc).astimezone() for local time with UTC offset
• Update all datetime.fromtimestamp() calls with tz parameter
• Store parameter cache timestamps as timezone-aware local time
• Parameter snapshot 'ts' field now includes UTC offset (e.g. '2024-01-01 13:00:00+01:00')
• Fix buggy manual timezone computation in AWG70000A driver
• Update all test files to use timezone-aware datetimes
• Update snapshot parsing to use fromisoformat() for ISO 8601 format

Fixes DTZ005, DTZ006, DTZ007 ruff lint violations. Existing datasets remain unaffected; only new ones have explicit timezone info.

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.09%. Comparing base (a997230) to head (adcc887).

Files with missing lines	Patch %	Lines
...rc/qcodes/instrument_drivers/Keysight/Infiniium.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8355   +/-   ##
=======================================
  Coverage   71.09%   71.09%           
=======================================
  Files         305      305           
  Lines       31913    31901   -12     
=======================================
- Hits        22687    22679    -8     
+ Misses       9226     9222    -4     

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