[Bug #21141] [DOC] Clarify what time is in UTC

	[Bug #21141] [DOC] Refine description of `Time#utc?`

	[Bug #21141] [DOC] Fix indentation

	[Bug #21144] Win32: Use Windows time zone ID if TZ is not set

	If the TZ environment variable is not set, the time zone names
	retrieved from the system are localized for UI display and may vary
	across editions and language packs for the same time zone.
	Use the time zone IDs that are invariant across environments instead.

	[Bug #21144] Win32: Convert the time zone name to the current locale

	The Windows time zone IDs provided by Microsoft as of 24H1 are ASCII
	only all, but the API itself is not impossible to set non-ASCII key
	name.  Prefer the current locale encoding for now until we move to
	UTF-8 including environment variables and command line arguments.

	Fix pointer incompatiblity

	Since the subsecond part is discarded, WIDEVAL to VALUE conversion is
	needed.

	Some functions are not used when `THREAD_MODEL=none`

	`rb_thread_sched_destroy` is not used now at all

[Bug #20327] Do not count subsecond to calculate UTC offset

	Assume that there will never be any time zones with UTC offsets that
	are subseconds.  Historically, UTC offset has only been used down to
	the second.

So that it can be referred as a part of the document of `Time`.

If we always return 0, we might as well not define
the function at all.

This drops the total size of a Time object from 86 bytes to 80 bytes.

Running the benchmark benchmark/time_now.yml, this commit improves
performance of Time.now by about 30%:

```
  Time.now
Branch:  13159405.4 i/s
Master:  10036908.7 i/s - 1.31x  slower

  Time.now(in: "+09:00")
Branch:   2712172.6 i/s
Master:   2138637.9 i/s - 1.27x  slower
```

It also decreases memory usage by about 20%:

```
ary = 10_000_000.times.map { Time.now }

puts `ps -o rss= -p #{$$}`
```

Branch: 961792
Master: 1196544

Co-Authored-By: Jean Boussier <byroot@ruby-lang.org>