BUG: Fix DatetimeIndex timezone preservation when joining indexes with same timezone but different units #22756

Job	Run time
Pyodide build	22m 52s
Linux-32-bit	18m 35s
Linux-Musl	16m 26s
python-freethreading	23m 49s
python-dev (macOS-13)	17m 54s
python-dev (ubuntu-22.04)	19m 47s
python-dev (macOS-14)	9m 12s
python-dev (windows-latest)	22m 46s
macos-13 actions-310.yaml	57m 27s
macos-13 actions-311.yaml	39m 58s
macos-13 actions-312.yaml	46m 1s
macos-14 actions-310.yaml	16m 57s
macos-14 actions-312.yaml	19m 23s
macos-14 actions-311.yaml	17m 31s
windows-latest actions-311.yaml	55m 22s
windows-latest actions-312.yaml	1h 6m 3s
windows-latest actions-310.yaml	51m 59s
Minimum Versions	24m 12s
Future infer strings (without pyarrow)	58m 55s
Locale: zh_CN	42m 10s
Locale: it_IT	42m 6s
Downstream Compat	6m 45s
Numpy Dev	17m 6s
Pyarrow Nightly	20m 34s
Future infer strings	1h 8m 19s
Pypy	1m 52s
ubuntu-22.04 actions-310.yaml	1h 6m 24s
ubuntu-22.04 actions-311.yaml	1h 1m 56s
ubuntu-22.04 actions-312.yaml	1h 7m 46s
ubuntu-24.04-arm actions-311.yaml	37m 8s
ubuntu-24.04-arm actions-312.yaml	1h 7m 29s
ubuntu-24.04-arm actions-310.yaml	38m 26s
	19h 3m 10s

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