date: fix -u flag to match GNU behavior for input parsing

[ChrisDryden]

There are a few test cases in the date GNU test and an issue that was made about the behavior that in many use cases the -u flag is not used and the behavior of the date implementation was not matching the GNU implementation when the date is passed as stdin and when when combinations of the -u and -d were used.

The main summary of the change is that parse_date takes in Zoned now which uses the same logic for every location where a date is parsed and it will use the -u flag if it is applied. There were also issues with the logic for parsing the timezone where: "Try to parse the date with UTC first to get timestamp" was used and this causes the uutils implementation to provide the incorrect date.

Fixes: #9018

[codspeed-hq]

Merging this PR will improve performance by 5.21%

⚡ 1 improved benchmark
✅ 283 untouched benchmarks
⏩ 38 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	file_tz_abbreviations	18.3 ms	17.4 ms	+5.21%

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_{Comparing ChrisDryden:date-utc-flag (798f97c) with main (b5a5c47)}

Footnotes

1. 38 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩