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

This reverts commit ff1a03cb8300527bda3f356fb0c7c48f4b97faea 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

[x] Did you update CHANGELOG.md with your changes?
[x] Have you checked our Contributing document?
[x] Have you ensured the PR clearly describes the problem and the solution?
[x] Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
[x] Have you checked to ensure that there aren't other open Pull Requests for the same change?
[x] 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 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 :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/NeurodataWithoutBorders/pynwb/pull/1908/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/NeurodataWithoutBorders/pynwb/pull/1908/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders) | `72.80% <57.69%> (-0.34%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/NeurodataWithoutBorders/pynwb/pull/1908/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders) | `83.96% <69.23%> (-0.11%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NeurodataWithoutBorders#carryforward-flags-in-the-pull-request-comment) to find out more.

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NeurodataWithoutBorders / pynwb