Fix copyright year to auto get current year

[wilsonwu]

The copyright year is still 2024:

after fix like below:

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[russellb]

lgtm ... one super trivial suggestion, which is to specify UTC as the time zone, instead of using the timezone of whatever system is building the docs (which may be UTC already).

I realize this only has an impact a few hours of the year, so it really doesn't matter!

[russellb]

Suggested change

	copyright = f'{datetime.datetime.now().year}, vLLM Team'
	copyright = f'{datetime.datetime.now(datetime.timezone.utc).year}, vLLM Team'