Revert "Do not auto-set timezone, allow date (#1886)"

[rly]

This reverts commit ff1a03c from #1886.

Motivation

DANDI is having issues publishing datasets without a timezone. While this is being resolved, the team decided to revert the change that removes automatically setting a timezone when it is missing. Partially reverting #1886 is likely to mess up the new tests, so for time efficiency, I am reverting the entire PR. We can revert this when the DANDI issues have been resolved.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Attention: Patch coverage is 76.92308% with 6 lines in your changes missing coverage. Please review.

Project coverage is 91.88%. Comparing base (05756f9) to head (7a8bce5).
Report is 16 commits behind head on dev.

Files	Patch %	Lines
src/pynwb/file.py	66.66%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1908      +/-   ##
==========================================
- Coverage   92.09%   91.88%   -0.21%     
==========================================
  Files          27       27              
  Lines        2643     2651       +8     
  Branches      689      692       +3     
==========================================
+ Hits         2434     2436       +2     
- Misses        139      142       +3     
- Partials       70       73       +3     

Flag	Coverage Δ
integration	72.80% <57.69%> (-0.34%)	⬇️
unit	83.96% <69.23%> (-0.11%)	⬇️

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